Merlin will be trying to get into doors that are farther away from the rooms, the smaller ones, and looking for either ones that seems to have simple locks - in which case he's going to see how much damage simply kicking it for a little while might do - or ones that look like they might be able to be pried open with something sturdy wedged into the cracks and enough force on the other end - which he'll devote far more time to. He'll have his staff, which has a blade on the end to jam in, but he's not above going and scavenging for different materials in order to jury rig a solution.