score:2
Although there are a couple of Alesia siege maps that show some Roman cavalry stationed outside the circumvallation line, I think you have a point that cavalry was somewhat restricted. From what we can tell from several accounts, however, including Livius and Caesar himself, the cavalry had to be able to counter threats from the Gauls within the siege lines, as well as Gauls of the relief army attacking from without--as the cavalry did at Mt. Rea by riding between the circumvallation and contravallation lines and then moving outside the circumvallation line. That fight was a close call, but it is clear that the cavalry had sufficient room for a massed attack on the Gauls.
Even though the cavalry appears to have been protected by the circumvallation line, which protection was required by the fact that the Roman Army was also besieged by superior numbers, the cavalry forces still had the ability to move at will beyond the fortifications. It is also possible, even likely, that small groups of cavalry were outside the circumvallation line in observation posts but still under the protection of Infantry and cavalry troops within the line.
Upvote:3
As the previous commentator noted, cavalry placed inside the lines would have been seriously restricted in their movements. Equally important, oppposing troops could have quite easily boxed in the cavalry by creating narrow corridors for them to pass through, and these corridors would have negated the blunt force of a cavalry unit in line of battle. At this time, it's likely that Julius Caesar would have thrown the cavalry into a battle first in order to split the attacking forces into small and disarrayed groups that are easier to destroy than solid masses of troops. The cavalry, then, had to have complete freedom of movement, not protection from a fortified line.
Cavalry troops, in addition to providing what we now call "shock and awe," are an army's eyes and ears and so must be free to go wherever they think an enemy might be massing against them--in this role, the serve as an early warning system for the Roman camp.