The Drawbridge was devised when moats took off big time until then the only way to cross a moat was to swim or to row across
In the beginning it was just a bridge and designed to be destroyed in the event of an attack it was usually built of heavy timber but this proved to be not so easy so another type of bridge was devised a typical arrangement would be to have one hinged edge so the that bridge could be raised up to make crossing difficult and also it provided a barrier to the castle entrants.
Archers would fire volleys of arrows at the attacking hordes, The bridge would be raised or lowered using ropes or chains, and an addition of slits in flanking towers to allow archers to fire volleys of arrows at the attacking hordes.
After a time around the 12th Century an additional protection was backed by one or more portcullises and gates behind the bridge. Now known as a drawbridge
By the 14th Century a bascule arrangement was provided to the drawbridge by lifting arms whose ends were linked by chains an example can be seen, at Herstmonceux Castle, the Normans also added a portcullis to the castle’s wall, at the end of the drawbridge. There would often be two portcullises to the main entrance
A Portcullis was a heavy timber or metal grill and this could be raised or lowered from within the castle, there would often be two portcullises to the main entrance. The one closest to the inside would be closed first and then the one farthest away.
This was used to trap the enemy and often, burning wood or fire-heated sand would be dropped onto them from the roof. Pouring hot oil is a myth; it was far too valuable to waste. An example of a typical portcullis can be seen on the British penny.