Polybius provides a detailed description of the layout and organization of a Roman Legionary Camp in Book VI of his Histories. Rather than try to rewrite everything in the section on the layout of a Roman camp contained in Polybius, I have just posted the actual text. I took the text from Bill Thayer’s excellent site at Lacus Curtis. He uses an older translation than the one I have at home, and it differs at some points. The only thing I have done to the text is edit it a little for clarity and added emphasis to some words, mainly putting most of the Latin titles in italics. I also include a diagram I made up based off the description in the book and the names for the different sections I found at the Internet Encyclopedia of History , which also contains a modified translation of the camp description. Polybius says it much better than I can.
27 The manner in which they form their camp is as follows. When the site for the camp has been chosen, the position in it giving the best general view and most suitable for issuing orders is assigned to the general’s tent (praetorium). Fixing an ensign on the spot where they are about to pitch it, they measure off round this ensign a square plot of ground each side of which is â€¢one hundred feet distant, so that the total area measures forty-thousand square feet. Along one side of this square in the direction, which seems to give the greatest facilities for watering and foraging, the Roman Legions are disposed as follows. As I have said, there are six Tribunes in each Legion; and since each Consul always has two Roman Legions with him, it is evident that there are twelve Tribunes in the army of each. They place the tents of these all in one line parallel to the side of the square selected and fifty-feet distant from it, to allow room for the horses, mules, and baggage of the Tribunes. These tents are pitched with their backs turned to the praetorium and facing the outer side of the camp, a direction of which I will always speak as “the front.” The tents of the Tribunes are at an equal distance from each other, and at such a distance that they extend along the whole breadth of the space occupied by the Legions.
28 They now measure a hundred feet from the front of all these tents (the tents of the Tribunes), and starting from the line drawn at this distance parallel to the tents of the Tribunes they begin to encamp the Legions, managing matters as follows. Bisecting the above line, they start from this spot and along a line drawn at right angles to the first, they encamp the cavalry of each Legion facing each other and separated by a distance of fifty feet, the last-mentioned line being exactly half-way between them. The manner of encamping the cavalry and the infantry is very similar, the whole space occupied by the Maniples and squadrons being a square. This square faces one of the streets and is of a fixed length of â€¢one-hundred feet, and they usually try to make the depth the same except in the case of the allies. When they employ the larger Legions (sometimes a Legion was 5,000 men instead of 4,200) they add proportionately to the length and depth.
29 The cavalry camp is thus something like a street running down from the middle of the Tribunes’ tents and at right angles to the line along which these tents are placed and to the space in front of them, the whole system being in fact like a number of streets, as either companies of infantry or troops of horse are encamped facing each other all along each. Behind the cavalry, then, they place the triarii of both Legions in a similar arrangement, a company next each troop, but with no space between, and facing in the contrary direction to the cavalry. They make the depth of each company half its length, because as a rule the triarii number only half the strength of the other classes. (Remember that there are only 600 triarii at all times while the principes or hastati could number 1200 or more men depending on the strength of the Legion) The depth varies so that even when there are unequal numbers of men, the length of the formation is always the same. Next at a distance of fifty-feet on each side they place the principes facing the triarii, and as they are turned towards the intervening space, two more streets are formed, both starting from the same base as that of the cavalry, i.e. the hundred-foot space in front of the Tribunes’, and both issuing on the side of the camp which is opposite to the Tribunes’ tents and which we decided to call the front of the whole. After the principes , and again back to back against them, with no interval they encamp the hastati. As each class by virtue of the original division consists of ten maniples, the streets are all equal in length, and they all break off on the front side of the camp in a straight line, the last maniples being here placed to face to the front.
30 At a distance again of fifty-feet from the hastati, and facing them, they encamp the allied cavalry, starting from the same line and ending on the same line. As I stated above, the number of the allied infantry is the same as that of the Roman Legions, but from these the extraordinarii must be deducted; while that of the cavalry is double after deducting the third who serve as extraordinarii. In forming the camp, therefore, they proportionately increase the depth of the space assigned to the allied cavalry, in the endeavour to make their camp equal in length to that of the Romans. These five streets having been completed, they place the maniples of the allied infantry, increasing the depth in proportion to their numbers; with their faces turned away from the cavalry and facing the agger(embankment) and both the outer sides of the camp. In each maniple the first tent at either end is occupied by the Centurions. In laying the whole camp out in this manner they always leave a space of fifty-feet between the fifth troop and the sixth, and similarly with the companies of foot, so that another passage traversing the whole camp is formed, at right angles to the streets, and parallel to the line of the Tribunes’ tents. This they called quintana, as it runs along the fifth troops and companies.
31 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 quaestor ium , and on its further side the rest of the equites extraordinarii are encamped facing the market, praetorium and quaestor ium . In the middle of this cavalry camp and exactly opposite the praetorium a passage, fifty-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 to 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 on all sides is at a distance of 200 feet from the tents, and this empty space is important 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. However, the 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.