Ehrenbreitstein
Additional views of the interior.  Ehrenbreitstein has, among other things, a youth hostel, a memorial to the fallen of the German army in previous wars, and a nice museum of artifacts.

The photo at left is the back side of the northern glacis protecting the heart of the fortress from land based artillery from that direction.  One of the ways to protect high-walled castles of the pre-gunpowder era was to build an extensive raised glacis in the areas of greatest vulnerability.  This glacis would deflect artillery shot and prevent any direct impacts to the walls.
Below, a diagram of Ehrenbreitstein, giving some idea of the tremendous size of this fortification.  There is parking on the north side, but for old times sake I preferred to take the seselbahn (chair lift) on the south side, marked with the long dotted line.

Here is the link to the official website for Ehrenbreitstein.
Tunnels, tunnels and more tunnels.  There are numerous tunnels and passageways in just the areas that are open.  There are more tunnels along the hillside, but these are closed.