"While 3 minutes is not alot of time to remain invisible, if cast on the right people in the right place, at the right time, it may prove useful. If we could also find a way to arm the slaves, that might give us the numbers and distraction we need to overwhelm their forces. There may be a chance that the fortress we would be invading, may have a weapons cache inside. Now that might also be risky since we dont know how strong these slaves are and if they we be of any use to us in battle or just end up as snacks, but if all elements fall into place, it could work. I would recommend splitting into teams, one group going to free the prisoners, another would rendezvous and arm them with stolen weapons, and finally the last group would keep watch, and hold off any resistance should someone be noticed and enemies come flooding in before we are ready to fully engage them. Then as a large attack force, go through the fort and overwhelm them and hopefully take them by surprise."