What level shall i enter oblivion gates at?

Any level you choose. It doesn't matter if you have any allies at all. I just went in there and cleaved the people with Umbra. Didn't even care if they saw me. I had the difficulty put higher than half-way too.

Any level will do for the Oblivion Gates. You're getting leveled enemies, loot, and possibly traps. (As in the damage they inflict.) I made a spell that combines flame weakness with fire damage witch cleaves through fairly decently.

I entered the gates when i was lvl 9 and had some trouble with the deeadric warriors so i used like 5 healing potions

I like to play them all through the game.
I figure the more Sigil Stones I get the better I am.They give the best enchantnments for low level person.
I am a level 22 and have closed 24 gates. Even if the enchantment isn't something I want, I enchant a ring and sell it.And you don't even need an enchanting alter to use them.

Sigil stones are the best part about gates really, the fact that the best stones will give you a massive enchantment without the use of alters, paying or even getting the spell. Get one of each enchantment and keep them and when you want to use it, use the duplication cheat (watch out though, it takes long), though I haven't done this myself because I keep getting the same type all the time.

I entered and closed the gate at Kvatch at level 2. All I had was a steel longsword with 10 pts fire damage and the arena heavy raiment with a leather shield.