The war theme in tower defense goes hand in hand with the basic TD game mechanic of towers doing something to enemies (mostly shooting them) until they cease to exist, and with the corresponding player feedback of seeing rapidly shrinking health bars and the like. Monitoring enemy health is a necessary performance measure that guides tactical estimates of whether the enemies are being hurt fast enough, tower construction and enhancement and use of special attacks.
Destroying enemies wholesale is the key element I'd try replacing, for example with simply stopping or delaying them (which allows player feedback to consist of enemy positions and status markers and timers, without health bars):
- Perimeter defense against mosquitoes or flies in a summer evening. Towers that create soft forbidden zones (e.g. perfumed candles), directional air blowing to push insects back, lamps to attract them, various types of barriers, and of course many lethal options like spider nets, automated laser arrays, zapping or sucking traps, insect-eating birds and insects, etc.
- A rather full-contact fantasy sport in which the offense team tries to have one of them go from A to B as fast as possible and the defense team tries to slow them down. Towers and attacks would hurt or daze or knock over enemies to disrupt movement, stop them into place, make them slide (into obstacles or off the road if possible), directly push or teleport them away, etc. Strategy would be about optimizing obstacles and making the enemy team pile up at choke points; a sub-game of attacking and repairing the towers would fit well with a team sport setting (the offense team fields specialists to support a few runners that are meant to reach the goal).