With lighting as the central feature, that suggests exploration should be the core mechanic.
Separating out vision from attack damage. Possibilities:
Use one color light for ambient lights placed around the level, and use a different color light for towers that do damage
Towers shoot actual bullets at targets within their light range. Bullets do damage, not light.
Death as a way of exploring. Each time you die, you drop an ambient light (or drop your flashlight). You respawn at the start of the level, but you don't lose your towers or money.
Level prepopulated with stationary enemies. You explore, open doors and build paths(?) - revealing the map slowly. You have to set up towers as defenses and traps.
Level starts with ambient lights that go out when the enemies spawn. Goal: get to the electrical breaker near where enemies spawn.
Devil's Tuning Fork style illumination of the level to reveal enemies at some interval
Thoughts from 2/26/17