Ehrenfels and the Mausturm
Ehrenfels is perched on the east slope of the river, with an excellent view in both directions, though it has a weaker view to the north.  While its primary weakness was upslope, the large curtain wall was to protect the inner part of the castle from missile attack from above.
A look inside of the castle (right).  The metal frame above is a walkway that was built into the castle years ago.
The Mausturm (Mouse Tower, below) is a small structure below Ehrenfels, and was designed as the immediate toll booth and lookout along the river, being built in the early 1200s.  At one point it was even used by the archbishop of Mainz to hide Church treasures.  Legend has it that when a local bishop robbed the people of their gifts to the Church, a horde of mice pursued him.  He fled to the tower, but the mice swam the river and promptly devoured him... hence the name!



A closeup view of the tower (lower left) shows a small door on the north side, but to my knowledge visitors are not allowed.
An earlier shot of Ehrenfels (left, c1979), taken from the highway below that follows the Rhein on the east bank.  At the time that this photo was taken, Ehrenfels was open to explore.  However, getting into the towers was very difficult, and a sign said absolutely no admittance to them!
An even closer view of the tower, taken with the telephoto lens on the Minolta A100 in 2007, shows a staircase going up into the highest observation point of the tower.  The Mausturm appears to be in reasonably good condition, but it is not open to the public.  Quite frankly, other than the basic tower and a stairway within, there wouldn't be too much to see.

However, the view of the Rhein from that vantage point would be fascinating!