I'd say at least Lv. 6, though Lv. 7 would probably be even better. I just went there today with my level 5 party and I had trouble even getting in. I would try my first time at Lv. 6 but if you have trouble, go back at level 7.
BTW, Find/Remove Traps is the most important thief skill in the game, IMO. Get that up high very early on so that you won't have to go around leveling up trying to get your Find/Remove Traps skill on your thief, believe me, it's a pain.
-Jafin
------------------
I am Jafin, arch-mage of the HADB clan!