Alter Aeon Level 21 Area Recommendations

While Kordan is primarily intended for players level 20 and below, a few areas near the upper range are suitable for higher levels:

The Caves of Mendelius and Qooriks Underground Lair
The lower levels of these should give reasonable experience up to level 21-23.
Gandor's Island, reachable via boat from the town of Naphtali I know less about this area, so im not sure if it's a really good exp zone.

The viking raid village. The village itself is the lowest level part of the area, so you should be able to handle the vikings there if you're careful and pay attention. If you can handle that without trouble, try the sea serpents in the ocean and the vikings on board the ship itself. Be hot and ready to flee though - if you get ganged up on, you can find yourself dead in a hurry.
The nurgling mound. Like vikings, this area is fairly safe as long as you stick to the edges and try to take out one thing at a time. Charisma helps here.
The dark forest northwest of dragontooth. This is a big open forest with lots of wandering mobs that are good exp for level 20. Don't go into the spider cave or the troll caves until you're higher level. The guards in the spider cave are pretty mean and will poison the crap out of you.
In redferne's palace, there's a pair of humming metal posts. Going through the posts (they are a portal) will take you to a military zone that should be decent experience. It's small, but has a lot of wierd stuff in it. I'd have it in your exp list and return to it periodically.
The djinni school, just outside of redfernes palace. This is a non-aggro area so the exp is a lot worse here, but some people like it because it's pretty safe. Be careful near the teachers, some of them are aggro. Do not enter the same room as the headmaster.

These are the archais areas I'm less familiar with, so I don't know how good they are as far as experience:

Areas on Kordan