Tower is active if there are no asteroids in target place and if it can be connected with source. (Placing tower on top of asteroid would make it to easy :P) Note that there is 50% chance of coin drop from asteroid, so luck is also a factor.
It only seems that everything goes slow and smoothly - game is not so simple, one small mistake can easly lead to chain reaction and lose :)
 "The Great Automate - Chapter 2"){kind=icon}
 "ReacTowers"){kind=icon}