Where did they keep the mules in garrison?

[Gaius Julius Caesar]

Therefore, a smaller number of mules would be needed, possibly, say, for moving supplies from the main forts to the milecastles etc, on Hadrians wall

An interesting thought. Of course, one of the reasons for building roads was to enable the use of waggons for heavy or bulky loads.

Yes of course, but then the wagons may have been utilized for the heavier duties of moving supplies up to the forts?
Or perhaps the wagons ran down the roads, the small garrisons along the wall went down to the road and picked up their supplies from the drop off points with a mule? A lot of possibilities.