Maybe a lemmings like affair, where you're trying to steer the flow of cute animals away from some disaster that they're all inexplicably drawn to.

Sounds more like you want a story/character then being in need of mechanics.

That said, for non-war you could make the towers refugee camps and the monsters refugees that need to be taken in before they overwhelm the cities

An amusement park, where you need to satisfy guests before they get to the end of the screen. Some rides are more fun (damage), are visible from further (radius), etc. If a line is full then the guests walk past it.

The immune system, where you fight off pathogens.

A time travel experiment goes awry and you need to transfer all the past and future versions of yourself back to their appropriate time before they escape the lab and ruin causality.

A conveir (not sure how to write this).

Or something like Mcdonalds. You receive bread (attacking) and place towers with butter, cream, sausage, ketchup. The result should be sandwiches or different types of foods: duners, sandwiches, pancakes etc and depends on how much food contains each of them on the exit you receive a bonus.

Haha. Seriously I think it will be interesting.

I feel like zombies in the media are starting to die off (hah), but there are always spinoffs of life simulation games popping up. Restaurants, farms, celebrities, etc. So your pizza idea might still work out or even something weird like a celebrity fending off paparazzi. But I think you have to differentiate yourself via the mechanics involved, the shop items, and how easy it is to progress in the game without buying anything. That's what will make it fun and differentiate yourself from the rest of the market.

I think there's two basic ways to go. One is to replace the basic mechanic of "whittle away at HP until the enemy ceases to exist" with something else entirely, like Lorenzo is saying. Especially "positioning", which is something you already need anyway.

• In general I like the idea of strategy games in general where misdirection and other positional hijinks replace the idea of "damage". For example, imagine a freeform TD where your towers spin enemies around, make them slip past an important turn, freeze them temporarily, catapult them over the base, stuff like that. Like say I'm defending something against mischievous but otherwise inoffensive creeps, like monkeys or something. I don't want to *murder* them for wanting my bananas or whatever -- they don't want to murder me, they just want a banana, throwing grenades is a bit of a disproportional response! But there's lots of comic mischief I could do in return.

The other ideas keep the basic mechanic and flip it to the idea of "benefit" rather than "harm", like trying to fill a satisfaction meter until the creep is fully satisfied. Some more themes for the latter, beyond the many good ones already above:

• Kaiju monsters approaching a city, but instead of killing them you're trying to feed them so full of enormous, house-sized desserts that they don't want to eat your citizens anymore.
• Something like Theme Hospital (but linearized and simplified like a tower defense), where sick people are streaming through your hospital and you have to try to cure each one before they leave the hospital.
• The above with more cute potential: scraggly dogs and/or cats travel through your grooming spa, and you succeed if they're gleaming adorable puffballs by the end.
• A refueling game like Airport Mania, but again linearized and simplified. Maybe robots returning to their birth factory to be recharged and occasionally fixed and cleaned. You're shooting electricity at them, the same way you might in a tower defense game, but they're enjoying it and leave when they're satisfied :)

Anyway, good luck! I'm always looking for more strategic or RPG-like games (casual or otherwise) that don't have a violence theme :)