It's basically an infinite wand hooked up to a big external power source to allow it at will use.  Most of the larger ironclads and floating fortresses tend to use them.  Most spell turrets carry a 5d6 damage fireball, though this big old machine likely carries a necromancy spell.  Most spell turrets are automatic, and fire at anything that comes within range when their defense mode is activated.  Most such turrets use visual targeting, though with this one, deathsight is likely.