Tower-defense
The main task of any tower defense free online game is to save your castle/tower with building the shooting protection against the ongoing hordes of attackers. They can be anyone: just regular warriors, invaders from other planets, robots, monsters from depths of oceans, magicians, ancient titans, and anything you would imagine. The goal is not to let them go through. It is half an engineer task and half – a business task. Why not war task? Well, killing attackers in the playing is actually the act of defense, which is not obligatorily done during the war actions. But it is about the wise location of your shooters and choosing wisely amongst the available options of spending money, which is scarce.