A portcullis is a grille or gate made of wood, metal or a combination of the two.
Portcullises fortified the entrances to many medievalcastles, acting as a last line of defence during time of attack or siege. Each portcullis was mounted in vertical grooves in castle walls and could be raised or lowered quickly by means of chains or ropes attached to an internal winch.
There would often be two portcullises to the main entrance. The one closest to the inside would be closed first and then the one furthest away. This was used to trap the enemy and often, burning wood would be dropped onto them from the roof. Pouring hot oil is a myth, it was far too valuable and rare at the time to waste as such. Also, archers could shoot arrows at the trapped enemies. There were often arrow holes in the sides of the walls for archers and crossbowmen to eliminate the besieging army.
By analogy to the gates, a portcullis is also the name given to a device used for quick change or scenes in theatricalstagecraft. The painted scene is attached to a grille, which then is made to appear in the stage, by moving it with a winch.