The gatehouse was a large frontal structure serving as the main entrance to the castle (any other entrance was very small, and was situated around the back or sides where there was less access). Prior to the 13th century, the gatehouse was somewhat of a weak point, but toward the 14th century, it's form grew in importance, becoming more mighty and complex than ever.
The portal usually consisted of a gothic arch deeply recessed between two flanking towers, from which it was commanded by arrow slits. Overhead, the portal was covered by a projecting timber hoarding called a bretasche. As with the curtain walls and towers, the bretasche later came to be made out of stone.
The portal itself consisted of an entrance was blocked on the outside by heavy wooden folding doors. Directly behind the doors was the portcullis, a massive wooden grating, usually 4-6 inches thick, framed and shod with iron. The portcullis was moved up and down in slots and was raised by a windlass in the portcullis chamber above. The doors swung inward, so if the portcullis was down, they would not open.
Behind the portcullis, the entrance way had a wooden ceiling (the floor of the above portcullis chamber) pierced with "murder holes" through which materials could be cast upon intruders. Guard chambers were also situated on either side of the entrance way which provided additional defense. At the end passage, there was usually a second portcullis, making the entrance way completely enclosable so intruders could be trapped.