They manage the night guards thus:
The maniple on duty there guards the consul and his tent, while the tents of the tribunes and the troops of horse are guarded by the men appointed from each maniple in the manner I explained above.
Each separate body likewise appoints a guard of its own men for itself.
The remaining guards are appointed by the Cs; and there are generally three pickets at the quaestorium and two at the tents of each of the legates and members of the council.
The whole outer face of the camp is guard by the velites, who are posted every day along the vallum — this being the special duty assigned to them — and ten of them are on guard at each entrance.
Of those appointed to picket duty, the man in each maniple who is to take the first watch is brought to the tribune in the evening by one of the optiones of his company.
The tribune gives them all little tablets, one for each station, quite small, with a sign written on them and on receiving this they leave for the posts assigned to them.
The duty of going the rounds is entrusted to the cavalry. The first praefect of cavalry in each legion must give orders early in the morning to one of his optiones to send notice before breakfast to four lads of his own squadron who will be required to go the rounds.
The same man must also give notice in the evening to the praefect of the next squadron that he must make arrangements for going the rounds on the following day.
This praefect, on receiving the notice, must take precisely the same steps on the next day; and so on through all the squadrons.
The four men chosen by the optiones from the first squadron, after drawing lots for their respective watches, go to the tribune and get written orders from him stating what stations they are to visit and at what time.
After that all four of them go and station themselves next the first maniple of the triarii, for it is the duty of the centurion of this maniple to have a bugle sounded at the beginning of each watch.
Walbank Commentary