The spaces behind the tents of the tribunes to right and left of the praetorium, are used in the one case for the market and in the other for the office of the quaestor and the supplies of which he is in charge.
Behind the last tent of the tribunes on either side, and more or less at right angles to these tents, are the quarters of the cavalry picked out from the extraordinarii, and a certain number of volunteers serving to oblige the consuls. These are all encamped parallel to the two sides of the agger,and facing in the one case the quaestors' depot and in the other the market.
As a rule these troops are not only thus encamped near the consuls but on the march and on other occasions are in constant attendance on the consul and quaestor.
Back to back with them, and looking towards the agger are the select infantry who perform the same service as the cavalry just described.
Beyond these an empty space is left a hundred feet broad, parallel to the tents of the tribunes, and stretching along the whole face of the agger on the other side of the market, praetorium and quaestorium,
and on its further side the rest of the equites extraordinarii are encamped facing the market, praetorium and quaestorium.
In the middle of this cavalry camp and exactly opposite the praetorium a passage, 50 feet wide, is left leading to the rear side of the camp and running at right angles to the broad passage behind the praetorium.
Back to back with these cavalry and fronting the agger and the rearward face of the whole camp are placed the rest of the pedites extraordinarii.
Finally the spaces remaining empty to right and left next the agger on each side of the camp are assigned to foreign troops or to any allies who chance to come in.
The whole camp thus forms a square, and the way in which the streets are laid out and its general arrangement give it the appearance of a town.
The agger is on all sides at a distance of 200 feet from the tents, and this empty space is of important service in several respects.
To begin with it provides the proper facilities for marching the troops in and out, seeing that they all march out into this space by their own streets and thus do not come into one street in a mass and throw down or hustle each other.
Again it is here that they collect the cattle brought into camp and all booty taken from the enemy, and keep them safe during the night.
But most important thing of all is that in night attacks neither fire can reach them nor missiles except a very few, which are almost harmless owing to the distance and the space in front of the tents.
Walbank Commentary