Theme: Medieval with green rolling hills.
Upgrades:
Weapon upgrades - Starts out with pistol, then progresses through a weapon tree like the one in the original HCC
Base upgrades - It starts out small with no walls, can upgrade towers, walls, extra floors.
Siege upgrades - pay for a tunnel to be dug through to the enemy base. It should be one way, so the people who buy it can slide down into it, and go out through, but no going back.