Enemies walk along the brown road. Stop them before they reach the end. You have 20 lives and each enemy that gets through costs one (a boss costs five).
Build: pick a tower at the bottom (Archer is cheap and fast, Cannon hits groups hard, Frost slows enemies), then tap an empty grass square.
Upgrade or sell: tap a tower once to select it, tap it again to upgrade. A fully upgraded tower can be sold for part of its cost.
Press Start wave when you are ready. Defeating enemies and clearing waves earns gold. Every fifth wave has a boss. Your best wave is saved.