How do I low the drawbridge to complete my ninja quest? I have already completed my castle mission and freed D'soto. Next question, which is better, ninja or samurai?

Ninja is the best advanced class short of the "special" classes, followed closely by the monk. Samurai is ok because you can learn Sun magic. You might want to think about spending some time as a Samurai to learn Sun and then switching to Ninja.

The switch to lower the drawbridge is in a small guard toward on the right side of the drawbridge, up the stairs. The Drawbrige itself is in the center of the north wall.

I also found the switch after completing DeSotos castle, I sure it's pretty common.