Saalburg- a Restored Roman Fort, and a look at the Limes
While Saalburg is not a castle in the sense of the word as understood in Germany, the word "castle" actually derives from the Latin "castellum" which would reference a fortified settlement.  Hence, Saalburg is actually a castle in this way. 

Saalburg is part of the ancient Roman "limes."  The word refers to a crossing path or boundary, and thus Saalburg was part of the borderland between the empire and the wilds of Germania.  The original fort was obliterated and virtually lost in history.  The foundation was excavated and actual plans for the site were found.  Thus, the reconstruction pretty closely follows the original design.
These photos of Saalburg were taken in 1978.

Various gates and courtyards have statues found at the site.  There is also a fine museum with numerous artifacts discovered at the site.
Below, a view of part of the restored courtyard. 
Left, another view into the courtyard and the covered walkway that links some of the buildings within. 

Below, a statue of Septimius Severus.
This view shows a work area within the fortified settlement.  Roman legions, when settled in occupation duties, actually developed their own crafts and basic products to help alleviate supply dependency on the empire.










A view into open areas of the settlement.  These open sections could be used either to drill the legions or auxillary troops, or to set up a tent compound to encamp any additional legions sent to the region temporarily to bolster the border.