I HAVE AN IDEA!Upgrades should upgrade health right?Well,once upgraded it should have a fence like this Lvl1:Nothing
Lvl2:Wooden fence
Lvl3:Stone
Lvl4:Spiked Stone
Lvl5:Steel walls
Lvl6:Spiked Steel
Lvl7/Final:Magic enrgy field surround building in a dome-like block