As you can see, it consists of a number of squares about which are buildings. The bulk of these buildings are actually just one building, consisting of long corridors on multiple floors linking to one another. It's possible to walk from the Hexagon (between L2 and L3 on the map) to the Computer Science department (L6 on the map) while remaining entirely indoors. You could go to M6 or L4 if you wanted to, and even the lecture theatre block if your definition of "indoors" includes "having something above your head if not necessarily to the sides".
When I was an undergraduate, we used to make the trip from Computer Science to the Hexagon (which at the time was the university's refectory) quite often. Occasionally, I'd play a game where I tried to get there without opening any doors. Many doors were left open, but not all, so it could involve changes of floor or trips down wings to strange departments like Sociology or Government. At the weekends, more doors were shut so if I played the game then I allowed myself to push on a door but not pull on any.
Surprisingly, I found out that some of my friends played the same game, developed independently. Occasionally, we would therefore get together and try to reach, say, the Physics Department (now the Law Department, from 4SE to M6) and end up taking excitingly circuitous routes and ending up knowing more of the geography of the university than we had any right to know. Sometimes, we'd get penned in when a passer-by shut a door behind us, and we'd have to wait for another passer-by to appear then all dart through the door before it shut (holding a door open for your friends was Not Allowed, although lifts were Allowed).
Although I'm sure this is a game that generations of students have played at Essex, I suspect its days are numbered. This year, the university has embarked on a scheme to make all external doors automatic, so that people in wheelchairs don't risk injury by pushing against them. This can only be extended to internal doors, which will doubtless be left open the whole time unless they're fire doors, in which case they'll be made automatic. You'll be able to get from anywhere to anywhere without opening anything.
I don't object to the installation of automatic doors (although they do use a lot of electricity), so I'm not complaining. If I were in a wheelchair I'd want them, so in that respect they're to be applauded. Still, it's not inconsistent to lament the loss off the old days, so I'll do that.
Lament, lament, lament...