Adventurer with nature + ice. Adventurer has decent item find/quality and no drawback, which makes it a good and low-risk builder for beginner. Nature has baby plant to quickly level up your carry and relatively easy to roll an end game carry (e.g. furbolg, jungle stalker, huntress, bonk, forest protectress, garden of eden). Nature also has skink to ensure a solid early game. Both nature and ice have tons of support towers to slow/stun creeps for crowd control. The three main ice towers for crowd control are frozen obelisk, igloo, and icy core. Frozen well and frozen wyrm are very good but they are unique towers so they're more difficult to roll.
Adventurer/Alchemist/Assassin with iron + astral. Iron element has good item find, thus is very powerful when used with builders with innate item find/quality such as adventurer and alchemist. Iron towers are mostly physical and decay attack type, so they generally struggle with Myt waves. With astral as the second element, it'll provide the necessary damage for Myt waves. Astral also helps to level up all your support towers to increase their efficacy.
Naga with darkness + ice. The increased attack speed and trigger chance from naga benefits many darkness and ice towers. A popular build is to use undisturbed crypt as physical carry and icy spirit as spell carry, both of which benefit from naga builder. Darkness has mostly decay and physical attack type, so the elemental attack type from ice helps with Myt waves. As usual, ice will provide support for crowd control.
Synergy Between Builder and Elements
After playing the game for a while, you'll notice that some elements work better with certain builders due to their synergy. Iron has good item find/quality so it works well with builders with innate item find/quality, such as adventurer, alchemist, royal assassin, maverick, backpacker. The increased attack speed and trigger chance from naga builder works very well with darkness, storm, and ice elements since they have many towers which could proc. Another synergy example is using antagonizer with astral, as the experience generation of astral towers will negate the low-XP malus of antagonizer. Spirit warden builder provides massive buff duration increase for your towers so it works well with nature and ice elements which has tons of support towers with stun/slow ability. Blademaster and barbarian builders have high physical damage and high crit chance/damage, which works well with nature and fire elements. Sorceress builder with her increased spell damage works well with darkness and nature. Benevolent witch builder with her increased mana regen works well with fire and nature as you have embershell turtle for early game carry and quillboar for late game carry. Royal assassin builder with its item quality and bonus damage vs boss pairs well with iron and nature. Trash heap from iron has good item find and bonus damage vs boss, so using it with royal assassin will help you kill boss challenge and increase chance of getting good item drops. Skink from nature is good for killing boss challenge, so the same argument applies. The more you play YouTD, the more synergy you'll notice between builders and elements.
There are 2 common locations for placing your towers: the start of the maze (often referred to as the 'U') and the 'L' corner. There are pros and cons for each location. The U is good for the following: counter gravid waves, fast game, and if your carry benefits from buff stacking (e.g. quillboar, gatling, polar bear). For beginners, it's recommended to build at the L corner since the creeps have to pass by your towers twice with minimal overlap between the first creep and the last creep. Once you're more comfortable with the game, feel free to try out building at the U and see which works better for you. Building at the U and L are both viable so it's just a personal preference. Apart from the U and L, there are other locations for tower placement but they are less common so won't be covered by this guide.
It's recommended to place your adjacent towers in a cascaded format (i.e. in a horizontal strip which allows 3 towers, offset the middle tower vertically by half a tower length). The end result of your towers looks like a honeycomb structure. Cascaded tower placement maximizes the number of towers affected by an aura.
There is a pinned message in the general discord channel which shows an example of tower placement at the L.
Basics of Carry Towers
There is a pinned message in the general discord channel which listed out potential carry towers for each element. Use that as a starting point but do note that the efficacy of carry depends on each game situation. For example, quillboar won't work well with barbarian builder, undisturbed crypt won't work well with realist builder, bonk/nerub/kraken works well if you roll them early, servant of the twin flames won't work well if you've split your physical and spell oils to two different towers up to that point, voljin/northern troll won't work well with maverick builder etc. The more you play YouTD, the better you'll be at assessing each game situation and deciding which carry in your stash to use.
Item find/quality is arguably the most important criterion in beating YouTD. You want to maximize the item find/quality stats of your carry, so you should always prioritize equipping your carry with item find/quality items. You want to equip your carry slots with mainly item find/quality items as long as you're not leaking. If you start leaking, then consider swapping out an item find/quality item for a damage item. A quick rule of thumb is to have 150% item find and 150% item quality on your carry by wave 150. That's on the low side but it's easy to remember.
Beginners often wonder whether it's better to use +DPS item or +% damage item. Generally speaking, +DPS item is good for early game and for carry with low base damage; whereas +% damage item is good for carry with high base damage. When in doubt, just try out each damage item individually and look at the carry damage stats.
Micromanaging your carry item slots is highly recommended. You can get away with it for non-extreme game but it's essential for a good YouTD run. Items which provide bonus damage vs size (e.g. bombshell, ninja glaive, gargoyle wing) or vs race (e.g. sucona, axe of decapitation) are very useful due to their multiplicative effect on the damage dealt. In comparison, regular damage items (e.g. beast head, heavy gun) provide additive effect to the damage dealt, which is less effective during mid to late game. You should always keep a stash of these vs race/size items and equip them in your carry when you encounter a troublesome wave. Items which change attack type (e.g. brimstone helmet) or deal bonus damage to unfavorable armor type (e.g. staff of essence) are also very useful. A 'strong', manashield, Lua armor, nature creep wave coming and your carry deals decay type damage? No problem, just equip sucona and staff of essence on your carry for that wave.
Micromanaging your carry item slots also helps with overall item find. Keep in mind that bonus vs race/size items typically provide the best bang for your buck in terms of number of slots needed. For example, one sucona slot for nature creep wave works better than 2 slots of heavy guns, so that allows you to have more slots on your carry reserved for item find/quality items. You should always make conscious decision to equip your carry with as many item find/quality items as possible.
In general, most players will go with solo carry or dual carry. Solo carry can either be a physical carry (only use physical oils) or a hybrid carry (use both physical and spell oils). A solo spell carry is also possible but not recommended for beginners. Dual carry is when you have a physical carry (which uses physical oils) and a spell carry (which uses spell oils). It is entirely a personal preference whether to go with solo carry or dual carry. Either system will work. Generally speaking, using a dual carry system makes it easier to get to bonus round; whereas solo hybrid carry could make you last longer in bonus round. For beginners, you can try out a dual carry system first due to the following advantages: more versatile against different armor types (e.g. a physical attack type carry plus an energy attack type carry will cover a wide range of armor types), less affected by 'protect' wave (if one carry is locked down by the 'protector' champion, the second carry can still kill creeps), easier to counter ethereal wave (e.g. magic attack type carry plus a non-magic attack type carry), and easier to counter xarm++ wave and immune wave (e.g. healing obelisk plus a spell carry). As mentioned, either solo carry or dual carry will work; so try out both and decide which suits your play style better.
Make it a habit to check the tower attributes window of your carry from time to time. It has a wealth of information such as damage, attack speed, spell damage, item find/quality, trigger chance, bonus vs race/size etc. All the numbers may seem daunting at first, but you'll get used to it over time and it's a very good way to assess the efficacy of your carry. Note that attack speed is capped at 500%, so if your carry is already at 500% attack speed, you can either apply swift oils on your second carry (if dual carry system) or on crucial support towers (e.g. frozen wyrm). Alternately, you can convert those extra swift oils. Bonus damage (the green number) is capped at 390624, so if your carry has reached max bonus damage, it makes sense to convert those sharp oils.
Oils, Consumables, and Item Conversion
You should auto oil your carry with seeker, swift, accuracy, sharp, aether, and tear oils. Sorcery, mana, and wizard soul oils are situational, depending on which carry you use. Keep a stash of them until you've decided which carry to go with, then you can decide to either use them or convert them. It's recommended to convert bounty, forcepoint, XP, and food consumables. There may be times when using XP oils are desired, such as using it early game with antagonizer builder. The flat XP portion of the XP oil is also useful to level up critical support tower early game, such as ray blaster. Experienced player can beat the game with less than 30 food, but if you're struggling with food, consume the plant or chicken consumables. Leave the pig and hobbit for conversion. Keep in mind that if you do a 2-to-1 oil conversion, you'll not get the same oil as you've used as recipe. For example, if you do a 2-to-1 conversion with hobbit and wizard soul, your outcome will always be either tear or aether.
It is always recommended to use the oils you need instead of converting them to a higher rarity, as you lose the effective potency. For example, 1 rare swift oils is not as potent as 4 uncommon swift oils. Some risk-taking players choose to convert all oils early game in hope of getting seeker oils, but that's not recommended for beginners.
With the exception of the backpacker builder, there are two types of permanent item conversion: 3-to-1 same rarity and 5-to-1 higher rarity. The formulas (note that randomness/luck is involved) for both types of item conversion are shown in the tool tip of the horadric cube. The formulas will help you to have a better prediction of the resulting item level range. It helps to pay attention over time to the approximate level range of items, so that you know what item level range to use as recipe if you want to cube a particular item. If you want to cube a high level item (e.g. panda dress, toy boy), you should be using high level items as recipe too. It's recommended to segregate out the high level items in your stash and use them for cubing when you have 3 or 5 of them. That way, it won't be hidden away in your item stash and accidentally being used together with low level items as recipe. For example, using a level 77 item and four level 3 items for cubing will be a waste.
The 3-to-1 same rarity conversion is helpful early game for crafting a +DPS item to help with troublesome waves.
Keeper of Wisdom Upgrades
If your builder level doesn't allow you to have all upgrades for keeper of wisdom, you'll have to decide what you want to upgrade. It's recommended to prioritize upgrading item find and attack speed. If you usually struggle around wave 60-90 and lose because you don't have food to build additional support towers, then pick food as your third upgrade. If you have no problem with food, then you can consider damage and trigger chance as your next upgrades. If you have more points left, it's up to you what you want to upgrade, but I would recommend leaving experience and mana regen to the very last (i.e. least important).
Buff and Debuff
Beginners often get confused between buff duration and debuff duration. Buff duration affects the duration of any buff (which could be a positive buff on another tower or a debuff on creeps) applied by the tower. Debuff duration affects the duration of any debuff applied on the tower. Think of buff % as an offensive modifier and debuff % as a defensive modifier. For example, increasing the buff % of cold obelisk will lengthen the duration the creeps are slowed, decreasing the debuff % of your carry will decrease the duration it's stunned by a stun revenge wave.
If you're using phantom to cast wind shear on your carry, the buff % of phantom affects how long the buff will last on your carry, while the spell damage % attribute of your carry affects the spell damage.
General Game Play Tips
It's advisable to start out the game with researching two elements simultaneously, instead of maxing out an element first. Maxing out an element could be useful for players who want a particular unique tower from that element early on, but chances are you're not going to get that tower. Simultaneous research on two elements allows you to cover a wider range of armor types (e.g. iron and astral make a good combo since iron takes care of Lua and Sol armor types, while astral takes care of Hel and Myt armor types). Researching two elements also allow you to reap the benefit from both elements (e.g. iron for item find and astral for XP, or darkness for item find and ice for crowd control). With the exception of a few, most towers upgrade requirements end at level 13 of an element. Therefore, after getting two elements to level 13, you can start on researching the third element. However, if you want to get at least one tower from that element after each wave, feel free to research it to level 14 or 15.
Your carry should be getting most of the creep kills. The rest of the towers are there as support towers. You can generally categorize support towers as buff support and debuff support. Buff support towers buff your carry and debuff support towers provide crowd control (e.g. stun, slow, entangle) or increase the vulnerability of the creeps (e.g. armor reduction, increased damage taken). You can use the buff group command so that the buff support towers with active cast will only buff your carry. Some notable tier one support towers include human sacrifice, paladin, valor, steam engine, and IT. Some support towers provide multiplicative bonus to the damage dealt by your carry so you should build them whenever possible, such as cursed ground, solar/lunar emitter, and sapphirons.
You can use the range checker to check the attack range and aura range before placing a tower. This is especially important for towers which need optimal placement, such as steam engine and quillboar. Keep in mind that some towers/items can be placed in such a way that you benefit from its bonus but not from its malus, such as essence of rot and holy energy.
Pay attention to the upcoming wave info. With experience, you'll be able to assess whether you'll run into trouble with a particular wave. For example, xspeed++ plus stun wave could be troublesome for alchemist builder, regen++ boss wave could be troublesome for helicopter zone carry, strong plus mana shield Sol armor wave could be troublesome for energy attack type carry, spellbinder wave could be troublesome for carry that relies on mana. You would want to equip vs race/size/armor items and damage items for these troublesome waves. Now is also the time to spend the gold and upgrade your towers. Some towers such as wooden traps benefit from timing your upgrade. If you upgrade your wooden traps immediately after its stun effect, you'll essentially get two stuns without waiting for the stun cooldown, which would be helpful for troublesome waves. There are various things you can do to counter specific creep attributes, for example, elune's bow counters xarm++ wave and ghost wave, staff of essence counters unfavorable armor type wave, building more mana towers reduces the chance of your mana-reliant carry being mana-burned by spellbinder wave, spell damage counters xarm++ wave, magic attack type counters ethereal wave, mefis counters immune wave, focus fire or equip crit items for regen++ wave, towers with regen reduction debuff counters regen++ wave, killing champion first counters protect wave, silence counters second chance and spellbinder waves etc.
Often times, it's possible to salvage a non-extreme game (at least to reach bonus round) even if you've had a bad run. Debuffs is your best friend in this case. Focus on support towers that could perma-stun (nature, ice, iron), reduce armor (fire, darkness, iron), amplify damage (e.g. buried soul, valor, marine), and increase vulnerability (e.g. bone shrine, sapphirons, solar/lunar emitters).
The best way to learn the game is to play more games and try out different towers and items. A good understanding of towers and items is essential in knowing how to deal with different game situations YouTD throws at you. If you have a game that is going well, try out different towers as your carry and see how each carry works. This will help you with understanding the limitation of each carry and the type of support they need. For example, you'll find out that gatling gun doesn't work if your trigger chance is too low, or stargazer doesn't work if you don't have sufficient slow, or voljin doesn't work if you don't have enough towers with debuff. Most importantly, have fun!