Place towers along a zig-zag path to stop the invading waves before any of them reach the bottom.
Tap any green cell off the path to place a tower (costs 50 coins). Towers auto-fire at the nearest enemy within range. Killing an enemy earns 10 coins; completing a wave adds a bonus. Each new wave brings more enemies, faster movement, and tougher health. You start with 5 lives — each enemy that escapes off the bottom costs one life. Run ends at zero lives.
Build at corners, not on straightaways. An enemy on a straight path passes through a tower's range once; on a corner, two range circles can overlap onto the same tile, doubling fire density. Look at where the path bends and prioritize those green cells.
Don't spread the first towers out. Two towers shooting the same chokepoint kill a tough enemy much faster than two towers in different sections. Cluster early towers at one corner near the path start, then expand later when you have coin to spare.
Let weak enemies through if it banks coin for the wave's strongest one. If a single big enemy is the last one alive and your range is full, a panic-buy second tower for it is usually too late. Better to read the wave preview and pre-build before the boss arrives.
Wave bonuses scale with wave number. The 50×N round bonus is significant in later waves, so it's worth ensuring you survive even at low life count — losing a life to bank +400 coins is sometimes correct.