I am reading Caesar's Gallic Triumph, Alesia 52BC by Peter Inker, and he provides a few maps of the camps and forts around Alesia. Two of the camps (camp H & K) are located outside of his cirumvellations lines, so they are completely exposed on all sides to Gauls. The only mention of the two in the book is that camp K housed Caesars cavalry and guarded the approaches to the river, and that camp H could also hold cavalry and guarded the northern plain and the Oze river. Evidence that they held cavalry has been found in later archaeological digs.
The locations of these camps just dont make much sense to me, for one they can be completely surrounded (especially in the case of H). Also as the majority of the Gauls were camped betweeen the Oze and Ozerain, so for the cavalry in camp K to attack the main Gaulic force they would have to cross the Ozerain first before entering battle, which does not seem very wise.
Caesar obviously knew that the Gauls would concentrate in the Plain of Laumes (which is why he had the stoutest defense there) so why would he put forts in these locations? what role did they play?
UPDATE: After watching a documentary on the subject where they reconstructed the fort from archaeological digs, they came up with this map, which clearly shows the cavalry camps connected to the main camps. I can't say which is 100% correct but this newer maps definitely makes more sense to me.