#2
Some tower defense games give you a build and a set of options. GemCraft gives you a chemistry set and tells you to figure it out. You combine gem fragments of different colors to create towers with stacked properties, then you optimize those combos over a campaign that takes well over a hundred hours if you want to see its edges. I tested this on my older laptop and it held up fine. The presentation is austere, especially coming from Bloons. No flashy heroes, no voice acting. Just a map, your gems, and an escalating horde. If deep optimization is what you are after, nothing else on this list touches it for pure buildcraft.