Simple. Well designed and easy to learn. But a recent update has turned me sour on this one. Progression was slow, but steady on an a system that gave individual units experience based off of random draws. Recent update multiplied the level system by ten. The cost to level units did not decrease. So factor random draw for experience on units that have varying degrees on rarity, and you end up with lots of walls to play with. It was fun to start. I do not find it fun any more.