Burr Tor is quite a large fort enclosing roughly 8 acres. On the ground there is not a great deal to see, there are faint traces of a bank and ditch toward the eastern edge of the fort.
The fort is now the airfield for the Derbyshire Gliding Club, they're alright about you sniffing around as long as you ask and stick to where they tell you to walk.
The gliding club offer flights for around £25, should give you a good view of the fort.
If I can add something to festboy's misc. notes below.....
Although once thought of as a hillfort, Burr Tor is now thought of as a stock enclosure. One of the reasons behind the rethinking is that in the 1780's some geezer named Cresswell described the earthworks as "neither broad nor deep."