hercufles: What lvl do you need to be to do that dungeon I never made it even against the battle horrors my party is average lvl 4, 5
I'd rather do it as early as possible, as the magical items found there can be quite useful. Or at least, do the first 2 levels as early as possible. I believe a level 5 team can do the whole tower with preparation and proper consideration before every major encounter. If you are skillful, a level 4 team with a level 5 mage and thief and definitely beat it.
A little spoiler: You will pick up the following items in the Tower
Plate Mail +3 (One of the best armors in the whole game)
Cloak of the Shield (One of the best cloaks)
Kiel's Helmet (Useful if you have Khalid)
Large Shield +2 (Best shield)
Bala's Axe (Useful against spell-casters)
Mace +2 (Best Mace)
Sling +3 (Best Sling)
Staff of Striking (A very powerful blunt weapon)
Flame Tongue (Best Longsword IMO)
Two Handed Sword +3 (Best Sword, is frightening even in early BG2)
In addtion, you will find tons of potions and ammo, but of course you will deplete lots of them during your tower adventure.