Stop the enemy from escaping by building defenses in cells around the exit point. Each cell can be upgraded and repaired. Easy mode is restricted to upgrading cells to level 2 only. Hard and extreme mode give more build cash but also feature much more enemies which also have more strength, HP and speed.. Good luck!
1 to 9 quick build in selected areaD = Delete selectedR = Repair selectedU = Upgrade selectedP = Pause menu open/close