The ruin of the former castle above Aach - today usually just called Old Tower - lies on the edge of the Eigeltinger valley on a plateau about 35 by 30 meters on a wooded elevation above the Aachtopf. A castle was probably built around 1100, but probably it was built in the early 11th century during the Salierzeit. The castle consisted of the partly preserved residential and defense tower and a ring wall. The builder of this small fortress, which serves mainly to secure the road - referred to in professional circles as Turmburg - was the free-noble family of those of Aach. Around 1200, the Lords of Aach, who had worked for the Hochstift Konstanz, became extinct. Before that, however, they had handed over their goods and rights in Aach to the Constance cathedral chapter. A destruction is suspected in the 14th century, at the latest with the Peasants' War in 1525. Today some of the remains of the site are still preserved. The southern edge drops steeply to the Eigeltinger valley, the remaining three sides are protected by a moat cut into the rock. Parts of the defensive wall as well as the tower, which is today called the old tower, have been preserved.