Standing on the Drawbridge                             -      -      - 
(-------------------------------------------------)     W <---(M)---> E
                                                        -      -      -

This bridge was originally constructed to help people cross the river
below, but it has been modified to block entry to the city as well.  Heavy
chains on either side of the bridge can be used to raise and lower it as
necessary, turning the river into a natural moat around the town.  To the
west is a solid rock formation that reaches all the way to the cavern's
ceiling, and the only way to get through seems to the the doorway at the end
of the bridge.  You could also drop down into the water below, or leave the
cavern to the east.