Ok, there are a few requirements in order to be able to leave Cyrodiil:

Have a $hit load of magicka (At least 300)
Have access to the Arcane University
Have a lot of health (At least 250 if not more)
Be at least an Expert of Restoration (Preferably Master)


Go to the Arcane University and make a $hit load of fortify acrobatics spells. Make all of
them last at least 30 seconds, or this will be one pain in the @ss. Make enough so that
you can get at least an acrobatics skill of 400, and be sure to make the spells last for
at least 30 seconds, depending on how many spells you made, you might need it to last
longer since it takes time to use all of the spells. It's fun to play around with, so you
can also use it to just run around Cyrodiil and jump over bandits and run away leaving
them confused.

Ok, now go to Anvil and use you Fortify Acrobatics spells and jump on top of the Mages
Guild. From there, climb your way onto the Chapel, and finally, jump out of Anvil. Now you
should be outside of Anvil in a glitch world. From anywhere in Cyrodiil you should have
access to Black Marsh, Skyirm, Hammerfell, Morrowind, High Rock, Summerset Isle,
Valenwood, Elsweyr, etc.

Unfortunately, it is all glitched up, but it is really fun to run around in, and great for
your Athletics skill if it's not already 100.

By the way, you need a lot of health for this because you'll be getting hurt very often
from all of the jumps. 

I would also highly suggest that you enchant a whole set of useless armor (maybe leftover
Dwarven armor from earlier in the game or something like that,) two rings, and a necklace
all with Grand/Black Soul Gems for Acrobatics so that you don't have to make too many
spells, and it lasts forever as long as you're wearing the armor so it just makes life a
lot easier. Easy increase of 80 Acrobatics skill.

Note: You will NOT run into enemies in the glitch world, you will not encounter any life
except for yourself and plants. Sorry, but you can't use a horse for this.

Have fun!<table class="table_ch"></table>