Buy a turret and army men to defend your position. Contains a large number of shop items to upgrade
Use your guns and grenades to kill incoming monsters before they penetrate your base and attack you. Make repairs, upgrade your wall and explore the base after you've eliminated all monsters.
Shoot the incoming enemies and use bombs if you have to. Upgrade your turret.
Keep the incoming enemies at bay while blowing them and their vehicles away. Upgrade through shop.