I beat the game on level 6. Bob the Builder is the only Hero worth having, and I don't know how the game is possible without him. Leveling up is totally pointless. I put all my points in gaining experience, only to never need it. I set my goal to getting Architect and Grandmaster Builder. More often than not, you are better off with more units during the first 2/3 of the level, then upgrading your units in the last 1/3.
 "Protector III"){kind=icon}
 "Hex Mines"){kind=icon}