Where did they keep the mules in garrison?

[D B Campbell]

... forts and fortresses, they seem quite full of buildings but often no or little space is labeled stables.

I think the question was about permanent army bases.

On the march, baggage animals had to be accommodated in the marching camp, as your quote from Polybius demonstrates. For the imperial period, Hyginus mentions a space allocation for mules. The soldiers' tents are erected in rows 30 feet wide: leaving a 6 foot gap as a lane between rows, the remaining 24 feet are made up of the 10-foot wide tent, a 5-foot wide area for stacking arms and equipment, and 9 feet "for the mules" (papilioni dantur pedes X, armis pedes V, iumentis pedes IX, fiunt pedes XXIV: Hyg., De munitionibus castrorum 1).