Well, I have two 'props' in the game already, and yes, the turret could indeed be a type of defense rather than a monster in it's own right. The generator could work as an explosive barrier, and I'm sure I could think of a "defensive" use for the teledrone... perhaps it could be an untargeted small barrier which follows enemy monsters about, hindering their movement and blocking the odd attack, which would play on it's indestructability and general lack of impact in the game. As for the last? Perhaps the speed boost pickup? Let the player fly even faster for a short while in exchange for mana?
EDIT: Well, I've implemented the Teledrone as a 50 health floating barrier which chases after enemies and soaks up a little fire before dying. This does have some down sides however, such as also blocking your own attacks. Likewise I've implemented the Generator, scaling it up to 2.0, and giving it twice the dimensions, making it 40 units tall and about 80 wide. It explodes very violently when it's 200 health have been taken away. Anything small would go splat, anything big would miss a chunk of health, anything really big, with +NORADIUSDMG wouldn't care. The turretfloor is already in there, and I'm now thinking about what else I could use as some form of defensive or at least passive thing. Perhaps a random powerup spawner? You fire it off, it spawns, summons a random powerup (could be armour, invisibility or speed boost, as they're all present in Virus).
EDIT2: Yep, with a bit of messing about I've now made a VirusItemSpawner, which uses the itemfog sprites from Virus (with new names) and can summon either a Shield +100, which goes up to a maximum of 300 for ZDoom Wars, or a Disruptor (partial invisibility) or a SpeedBoost, which is the equivilant of the Boots of Speed I believe. Now it's back to just monsters to sort out
