Drag a tower card onto the grass to build it — or tap the card, then tap the grass. 1 2 3 4 pick which tower too. SPACE sends the next wave.
Click a tower you've built: U upgrades it, S sells it back. F is fast-forward.
Arrow Tower — cheap and fast, hits flyers.
Cannon — huge splash damage, but can't hit flyers.
Frost — barely hurts, freezes everything nearly still.
Lightning — jumps between monsters and ignores armour.
Tanks have armour, which takes a chunk off every hit — so lots of little arrows are useless against them. That's what cannons and lightning are for. Bats fly straight over the road.
The waves never run out. A boss turns up every fifth one.