30 is sorta low level when you think about in.

If you look at the SP curve and time requirements 1-24 can litteraly be done in a day or two in leve partys and alittle grinding. 30 can be gotten to in about 2-3 more days. (1-30 is only like 20-25% of the way to 50 in terms of overall sp).

Then again it is in the middle of 1-50. So it is all how you look at it. but I think 30 is a good place for a dungeon. That level is easily achievable by players in a rather limited amount of time.