This view shows the inside from a wider angle. To the left are the two granaries (the museum), to the right the commander's house (praetorium). It is a later addition and therefore has the correctly painted walls. It houses rooms for the staff working in the museum today. I'm pretty sure the original granaries didn't have such large windows, but I suppose some concessions to the modern use had to be made.
One of the features the Roman border fortresses share is the combination of a stone wall - surrounded by additional ditches and earthen walls - with an earthen rampart on the inside that also serves as battlement. The reconstructed fortifications of the German Saalburg fortress present a good, if rain blurred, example.
Walls and crenellations seen from the inside
I suppose this unsual combination goes back to the history of Roman fortresses. They all began as semi-permanent structures with earthen walls and timber palisades on top, a more elaborate version of the marching camps.
Along the frontiers (the limes Germanicus, the Hadrian's Wall, the Syrian limes, and the Welsh forts) the fortresses were later rebuilt in stone, most of them in the 2nd century AD. Besides the stone buildings inside the forts, the defenses of earthen walls and trenches got an additional stone wall, watch towers, and stone gatehouses.
Walls seen from the outside
There are no trenches here today because of the situation in the middle of a town. The shots throught the trees and foliage look romantic, but of course, in Roman times trees would not have allowed to grow so close to the fort and give some sneaky Germans the chance of cover. There are different estimations how large the cleared areas were and it might have differed in different countries, but a minimum of bowshot distance can be assumed.
The ramparts added to the stability of the walls, definitely well enough to stop a ram, and neither the German nor the British tribes had any more elaborate siege engines. They usually tried to climb the walls or breach the gates only to meet with pointy pila poking at them. Attacks on fortresses were not very frequent; the tribes prefered to attack the Romans outside when they were stretched out in marching columns.
I bet after walking a day with full equipment through bad weather, Roman soldiers would have been very glad to see those walls. Probably after sweating under a hot German summer sun as well.

I'm a blogger from Göttingen, Germany, with a MA in Literature and History, interested in everything Roman and Mediaeval, avid reader and sometimes writer, opera enthusiast, traveller with a liking for foreign languages and odd rocks, photographer, and tea aficionado. And an old-fashioned blogger who refuses to get an Instagram account.
(See here for Archives for mobile devices)