Alsbach
Alsbach is often overlooked, because it is overshadowed by the larger Auerbach nearby.  It was initially constructed in the early 1200s by the Lords of Bickenbach (which is possibly its original name), and is situated just south of Darmstadt above the town of Alsbach.  The castle became a joint fortress owned by several families with land holdings in the area, and was eventually burned by the Dieburg family from the Frankfurt area in 1463.  The castle was rebuilt in 1504 and came under control of the House of Hesse.  During the 17th and 18th Centuries the castle slowly began to decay as it was abandoned, like many other castles, as too costly and unnecessary to maintain.  Today it is a preserved ruin.

These photos were taken in the winter of early 1978, the structure completely deserted at the time, which made for a fabulous excursion (though it was very cold that day!).
Above, the main structure of the castle is in a triangular shape to provide maximum defensive benefits at the cheapest construction cost for that time frame.  The beauty of Alsbach is that it is largely untouched since its decline, and thus the structure is an authentic example of a fortification from the early 1500s.

Below, another view of the main keep from just outside the walls.  The castle has an outer and inner ward which enhanced its defensive features.
Above, the main entrance that takes you into the outer ward.  The outer ward was protected by a dry moat as well as the primary walls.

Below right, the main keep showing the small walkway (a modern addition of course) into the tower.  The keep is extremely thick (my recollection estimating perhaps 12-15 feet) with a VERY narrow winding stone stairway taking you to the top.  The view through the Rhein valley is magnificent, but when I took these photos the sun was almost gone as dusk comes early in more northern latitudes.  Thus there just wasn't enough good light for the photo.

Below, a view inside the inner ward.  There appears to be subterranean levels but the access points are blocked off.  As of this time (early 2008) I do not know if these areas have been excavated and explored.