Other forms: drawbridges
In olden times, if you lived in a castle, you might have a drawbridge that could be raised and lowered depending on whether or not you wanted to let people cross your moat.
A drawbridge gets its name from the fact that it could be "drawn up," or raised, to keep intruders or unwanted visitors away from a tower or castle. The typical medieval drawbridge spanned a deep, wide moat (a trench filled with water). This moveable wooden bridge was usually attached to a guarded gatehouse and could be raised and lowered fairly easily with ropes or chains.