The Ketzerturm was the first of the town's three large defense towers to be built and has been preserved to this day. It was erected before the city wall was built (1448-1483). The tower is characterized by its high entrance, which was only accessible via a wooden staircase.

The heretic tower is a round tower with a cantilevered platform surrounded by a stone parapet. Several water reservoirs can be seen around the tower. These are popularly known as "Pechnasen", but were used to drain rainwater. The middle of the platform is characterized by a brick cone, which served as an exit to the platform and also protected the defenders from bullets.

The second tower entrance, which has been preserved in its original state, is 6 to 7 meters above street level to the east. In wartime, a ladder was used to climb up, which was then pulled into the tower.

The construction of the tower is very carefully executed. Inside, you can clearly see the structure of a late Romanesque keep with the function of a small castle. The heretic tower consists of three storeys, whereby the first floor can only be reached through an opening from above. This opening is referred to as the manhole or the "Angstloch". The ground floor could only be reached through the manhole using a ladder or a rope. According to tradition, heretics were imprisoned here. They gave the tower its name.

 The second floor is accessible via a door that can only be opened or closed from the inside. Two corbels are set into the outer masonry in front of this door. The first floor was lit by various small windows, which also served as embrasures. The room is separated from the next floor by a massive cupola, which is also only accessible via a manhole. From the 2nd floor, a ladder led up to the topmost defensive slab, which also forms the roof of the storey. The exit to the defensive slab is located in the conical stone dome. This dome is also the top of the tower. 

In the 1980s, the Tennstedt Heimatfreunde created an entrance to the building adjacent to the tower at the base of the tower. The so-called „Haus am Turm“ was opened as a museum in 1986. Unfortunately, it is now closed, but is to be revived.