During WWII a number of AAA towers were built by the Germans. The purpose of these towers was to raise the guns above the surrounding buildings and given them a better shot at allied bombers. The towers were built with 3.5m reinforced concrete walls and were considered invulnerable to bombing. Right there are two major flaws in the plan: 1. Even if the guns could see the bombers, they lacked the range to get a shell anywhere near them, and 2. the allied grand slam[a] bombs had broken much thicker reinforced concrete.
Despite this the towers were built, and they duly proceeded to have next to no influence on the war.
Post-war however, it turned out that they were next to impossible to demolish - the permission to drop grand slam bombs for civil engineering purposes probably being denied by the authorities. The towers were kept as memorials. (As a sidenote, the original Nazi builders had envisioned this as well - although they had expected the towers to be memorials to the victory of National Socialism and not to its defeat.)
Close up of the gun tower. Each AAA tower complex consisted of two towers, the gun tower where the AAA guns were fitted, and the fire control tower.
The fire control tower.
Access to the tower is forbidden, but it still houses some birds of unknown political affiliation.
The gun tower seen at a distance.