Defend your core (the blue crystal in the middle) against eight waves of raiders. Between waves you get a build phase: gather wood by shooting trees, then build walls to slow the raiders down. Raiders chew through walls, so keep repairing and keep shooting.
Shooting: drag with your left thumb to move; touch the right side to fire (it aims at the nearest raider automatically, or drag to aim yourself). On a computer use W A S D, the mouse and the left button (or space).
Building: press the Build mode button (or B), then tap a square on the map to place a wall for 10 wood. Tap a wall again to remove it and get 5 wood back. Wood also trickles in over time, and every raider you defeat drops a little.
If the core is destroyed you lose. If you are knocked out you respawn after three seconds.