Sidescroller tower defense...

Sidescroller tower defense where your units move automatically and stop when they detect an enemy or base on their range
To summon units you need money, after that they start charging. You also have a max money limit so you cant save up infinitely

You get money by killing enemies or by reaching certain thresholds percents of HP depending on the enemy as well as knocking them back 

Simulate the best ways to handle the economic design if there are supposed to be cheap, mid and expensive units