In ApoDefence you can build 5 towers and have to eleminate 20 enemies. You can choose between different levels, you can upgrade your towers and research new technologies to build better towers or to have more armor etc. You see you have to do many things.

Controls: You can do everything with the mouse. Left click to select something and right click to deselect it. There are also some shortcuts: Press 1 to 5 to build the towers etc