You simply go to Disney's website and type in your dates, it will give you the availability of the levels at the campground. The campground map shows the loops and levels.
As everyone stated above on can request a loop but one may or may not get it. We are at 33 percent for our stays. We always call and let them know what we are coming in with, a 43ft trailer with six slides, so they can put us in a big enough site. The Fort from number 1 to number 0 as the crow flies is approximately one mile.
With our setup, we are very limited now as to where we fit, so I know where we will be on further visits.
The Fort does site assignments about 3 days before you arrive, based on sites available, lengths, requests, length of stay, etc. The computer does some of the work, then a CM has to go thru and fine tune it, bless this person.
We have stay in 100, 1400 and 1100, and well they were all good loops, after all, we were in Disney on Vacation.