Please read all of this. :D Tap on enemies to defeat them. It's that simple. :D Red - weakest, blue - decent, green - strongest. The rainbow one does 10% damage and cannot be killed unless touched by an active wall. Buy walls to block them off, and maintenance to assist. (Tip: When placing a wall, before securing the location by clicking, press "E" or "R" to rotate.) Also, you should buy a tower because it's overpowered
The goal is to reach 10,000 coins to beat the game, with all 6 walls and one tower. Maybe you don't have time to learn about Rome if you're busy dealing with enemies.