I'm on Memory Block 3, in Damascus for my first time, and I killed the doctor abusing his patients (and turning them into mental cases) in the castle. I returned to the guild, and now I have no objectives. What do I do?
You have to head back to Masyaf now and speak to Mualim. You have to do this after nearly every assassination, except on the last missions in memory blocks, after which you return to the real world.