There are 4 core towers to construct in Alien Creeps TD:

• Infantry: Build a barracks and the troopers inside will attempt to stop and slowdown any Creeps coming towards them. Be sure to move them into Creeps way!
• Machine Gun: A fast firing direct damage tower, cheap to construct and fast to jump between targets.
• Laser: Fires a focused energy beam with high damage but long reload times. Highly effective against Shielded enemies.
• Rockets: Slow firing but each rocket packs a punch damaging any units in close together with its splash damage.