how do i open the drawbidge at shurugeon castle?

have you tried the "back door" to the castle (ok back hole)? If so check the tower next to the gate (not sure which side)

There is a lever in one of the sentry towers.

If you are on the Ninja quest be sure not kill ANY of the ghosts. If you do, you will have to go back and try again. Took me about three times to figure that one out.

hmm, played the ruins twice, and as my warriors had finished advancing to the sentry towers the ruins saw no single ghost 'alive' no more. even though, the Bushi Dojo promoted my cat dude to a ninja. Looks as if the game decides at random whether or not the quest is completed

p.s.: i encountered this nasty "stuck on stairway to drawbridge-lever" bug. I circumnavigated this inconvenience by climbing up the walls of this lil ruin west to the drawbridge towers and jumping over to the main castle walls. should be ok, even with a minimum of Athletic points