You play as the bad guys in this tower defence. There are often multiple paths and multiple exits to defend. If an enemy reaches the exit, you lose a health point, or 2 for the stronger enemies out of your total of 20.
Sometimes there’s areas where enemies also spawn in, like from huts/caves etc which you also need to watch out for. Some of these can be fairly close to your exits which means you need to quickly dispatch them.
You earn gold when you defeat each enemy, which you then use to build more towers, or upgrade existing ones. Upgrading often just increases the attack strength, but some increase range too. On the max level, there are then 3 bonus abilities you can unlock and sometimes upgrade a couple of times.
You can only place your new towers in a few set positions, and many of the map designs mean you are weaker down a certain path due to limited slots. On some maps, you might have to pay a small fee to unlock a slot, or sometimes it can be restricted e.g. vines that regrow on that slot and slow your attack speed.
There’s basically magic and physical attacks and each enemy can have protection from those. You also get flying enemies that only some towers can attack. So you have to use different towers to make sure you can defend adequately.
Some towers can deploy units (some are just barracks so deploy only, but others can attack too). The units will attack enemies that enter their circle. Although you can change the rally point, this does not change the attack range at all; which means you often see them just standing idle even though an enemy is really close to them. These units are great at slowing enemies down because they will always engage them before moving on. These towers will produce new units on death but will only have a limit of 2 or 3 active.
You have 1 hero unit who can freely move around the battlefield. New characters are unlocked as you play and also level up with skill trees of their own. They have a few special abilities that they automatically trigger and some that you manually trigger on cooldowns. These often feel essential to use and makes the game feel more active than other tower defence games.
There’s loads of towers which are unlocked as you play, but you can only bring 4 of them in which are selected before you select the level. I find tower defence games like that often feel like they have larger difficulty spikes because it is hard to balance them equally leading to certain favourable strategies. There’s the heroes, and a separate upgrade tree which you will max out towards the end of the game, but it's another thing that can make the difficulty feel unbalanced.
There is no way to increase the game speed, so the levels often feel a bit too long to replay and is therefore frustrating when you fail and have to replay them again. There is often some opening dialogue which cannot be skipped which adds to the frustration. The only way to slightly speed up the levels is calling the waves early which gives you a small gold bonus.