I don't quite understand why there is not the ability to remove defenses so other defenses can be placed on the same square. Sometimes a level is completely lost and needs to be restarted because you place a barricade instead of a mana crystal etc etc. Mistakes happen on small screens. Having to wait for a hero to destroy something so you can replace it seems strange.
It could be at object or creature that takes time and costs mana if necessary to preserve balance.
