From a game design perspective, I've got the following idea:
Traditional Tower Defense game. - an open, mazeable field as monsters approach the castle.
"Towers" consist of ground-pounding units like foot soldiers, moats, and other things. yawn
However, I'm adding the following features:
"Town" - a separate screen for upgrading units and buying new abilities.
"Tap Defense" - There are archers sitting on the wall with excellent range - tap monsters to shoot them. RoF, Damage upgradeable in town.
"Tap Defense+" - for cash purchases, you can buy wizard spells that deal mega-damage (think Eagle in Angry Birds).
"Underground" - the paid version will come with Dwarven Sappers, which give you an additional screen of "mazing" and towers "underneath" the current map, so you can build defenses in at least 2 levels.
Is mazing, tapping, and 2 z-levels too much?
What kinds of things should be considered? I already know that pausing while placing structures will probably be needed (or wanted). Anything else?