Ah yes - the tilting bridge!
When you fall off the bridge you go West where you have additional options - if you go to the North you have 2 paths. The NW path will lead to a room in which you will find a key and a back door leading to the swinging/poisoned water puzzle room (bad description, sorry). The NE path will lead you to a maze. IF I recall correctly - it will be solving this maze that will lead you to the other side of the tilting bridge and a button to stop it from tilting.
If you take the South exits you have a choice between the SW to the werewolf or the SE to the Vampires. Eventually these paths will lead you to a section of the castle which is the lower level below the entrance area.
It would have been in that area that you found a gate you could not open. To open that gate you need to visit G'Ezzered Ra - and to do that you need the key you find in the NW path from the tilting bridge.
GRa is to be found by (from the entrance) going up the stairs - opening the gate, traversing the large room with the ghosts etc., up the stairs at the back, follow the corridor until you come to stairs going down to a closed door. Use the NW key on that door and you will find G Ra!
Hope that helps!
Wyv