Enemies walk along the purple path. Every enemy that reaches the end costs you lives. Build towers on the cyan tiles to stop them.
Building: click a tower in the shop (or press 1-0), then click a free tile. Hold Shift to place several. Right click or Esc cancels. On a touch screen, tap a tower, then drag on the field and let go to build.
Upgrades: click a tower. Each tower has two upgrade paths with four upgrades each. Only one path can go past the second upgrade. Every upgrade changes how the tower looks. Point at an upgrade (or tap its card) to see the new range on the field. The Towers page shows them all.
Targeting: First, Last, Strong or Close. Change it per tower.
Waves: the panel on the right shows what comes next. Watch out for Ghosts (need detection), Armored (need big hits, poison or fire) and Shields (need many hits).
Keys: Space starts the next wave, P pauses, F changes speed, U / I buy upgrades, T changes targeting, Del sells.
Modes: Classic ends after wave 40. Endless keeps going with ever stronger waves. Sandbox gives you unlimited money and lets you pick any wave. Your game is saved at the start of every wave, Continue brings you back.
Stars: one for each difficulty you beat a map on. Beating Hard unlocks Impossible on that map.