Question About Tower Of Izyles

[Lirange]

Yes, i've already read the guide, but i still have some questions about it.
Okay so its a dungeon, but you can only enter it with a party? So every server can only have on dungeon raid going on at one time? Or is it like a kq where there are multiple times you can enter it. And the lvl range is quite high. Would a lower lvl, like 20-30 be useless in the dungeon?