The Donau (English: Danube) River is a absolutely beautiful, but there are not too many castles along the river worth seeing. However, it must be noted that there are a few that are not listed on this site as I've not yet been to them. Moreover, there are some good ruins along the stretch of the river from Sigmaringen to Tuttlingen, but these require an extensive investment of time and energy to pinpoint them and hike to them.
Below right, the roads along the river and going into the valley have a number of tunnels carved into the rock. These make for an enjoyable and interesting drive!
Below left, the Gebrochen Guttenstein just west of Sigmaringen is an example of most of the castle structures along the river. While fascinating to look at, there is not much there to discover. Nevertheless, the lure of such ruins is that few people ever visit them and that they have been largely untouched since they began to fall apart hundreds of years ago.
