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?