This fine figure of a wall is the Bürgerwehr (“citizen defence”), part of the late Medieval fortifications instigated by Prince Archbishop Berhard von Rohr. Up until 1816, the area was patrolled by the Bürgergarde, a local militia.
You’ll notice that the original masons used small rocks. These walls were built to absorb the impact of medieval armaments. Once gun powder-powered artillery and cannon arrived on the scene, bigger blocks of stone were required. You can see an example of this more modern construction technique in the Müllnertor Gate at the north end of the park.