If you can get through the sewers ok, you should be able to reach Zandalor. I did most of the quests in Rivertown and Verdistis and cleared the Rivertown sewers before entering the castle, and didn't have any problems. A character level in the mid to high twenties should be fine. At lower levels it is more difficult, but not impossible.
During the actual rescue, Spell Shield can help with the traps in the sewer treasure room. I maxed this skill with purchased spell books (
Free skills, and spell books you can buy spoilers). If the traps are killing you, check your resistances, and install the
patch if they are negative (actually, you should probably install the patch anyway if you do not have it, or if you didn't get a re-release version that was already updated to the latest v1.0034A version, as shown in the bottom right corner of the main menu).
If you are having problems with the group of orcs, and hit and run tactics do not help, there are 3 shadow potions on the table in the room just before them, that you can use to bypass fighting them completely.
