PCs get caught in hole (old castle, cave?) with overwhelming numbers against them. They have some warning and a time period when they will be relieved if they can hold on. Idea is that PC improvise with what is around and hold out for siege. Turns the GMing on its head. They have a plan of defenses, not the GM, and GM leads his baddies against it.
Players spring their surprises in traps etc. Must have a map agreement on what can be done in time available. Players tend to cheat outrageously but great fun for all concerned with a change of pace for both GM and players.