Drawbridge

There are three drawbridges in NetHack - one in the Castle, and the other two on the Valkyrie quest. They are widely considered extremely dangerous, since if standing directly in line with a drawbridge when it is destroyed, you can be hit by a flying chunk of metal which will kill you instantly.

A closed drawbridge is represented by #, and an open one is represented by ..

Commentary

The door beyond the moat of Fort Ludios is not a drawbridge, but an ordinary door.

Strategy

The castle drawbridge is opened by playing the passtune on a musical instrument. The passtune can be given to you by your god while praying on an altar, or derived by playing a game of musical mastermind right in front of the drawbridge.

Once you have the passtune, the drawbridge can be opened and closed at will in one turn, by playing the instrument again. Anything standing on the drawbridge will be crushed or drown, killing it instantly. This is thus an excellent way of clearing out the hordes of monsters in the castle courtyard. Do note however that this will destroy any possessions carried by the monsters you clear out--and the monsters in the castle are an excellent source of loot.

Alternatively, with a decent source of opening and locking (wands or wizard lock + knock are good sources), you can repeatedly open and close it without even knowing the passtune.

Some players prefer to destroy the drawbridge as soon as possible, from a safe distance, to eliminate the chance of a Soldier with a wand of striking hitting it at random and sending the huge chunk of metal flying towards you. In any event, never stand in line with a drawbridge, whether open or closed, in a combat situation. Drawbridge death is irrespective of hit points or armor class.

Reference

dbridge.c contains the source code for handling drawbridges, which runs to some 940 lines--a real tribute to the kind of devotion to minute detail that characterizes NetHack.