Battles are WAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAY too dependant on gold. I have stockpile of around 600 each of Iron and Crystals, but I never have enough gold.
Level 15 is jsut impossibe, the only way to beat the bosses is to give brute force against them, since they don't reproduce, but wehn YOUR units are randomly moving all over the place and not automatically attacking either the bosses or the spaces, it's impossible to keep them alive.
I found an odd glitch:
When you purposely block the spaceships on a level, where it would otherwise work, and then remove the block, the level tries to finish, but can't because you are holding a piece.