The former Imperial Palace (Kaiserpfalz) in Kaiserslautern was built in around 1152 by Emperor Frederick I Barbarossa. The only remaining parts of the Imperial Palace are a few carved blocks of red sandstone and remnants of the stone outer wall of the double chapel added by Frederick II.
Following the excavation of the rest of the Imperial Palace and the evaluation of the findings, the city mounted the historic foundations. The Domus (imperial building) originally measured 25 by 19 meters and was 19 meters high. In order to get a real sense of the size of the building, a 3D steel and tamped concrete construction that imitates the size and shape of the Imperial Palace was built.
You can see the remains of the castle wall, which have now been fully uncovered, just next to what was the Imperial Palace. The Stapf fortress was also rebuilt in this location. The fortress was erected at the beginning of the Thirty Years’ War, in around 1619/1620, by fortress builder Adam Stapf on the orders of the widow of Frederick IV, Elector Palatine of the Rhine. Stapf built primary defensive walls with trenches, bastions and curtain walls which protected the medieval castle and city walls.References:
First record of Kastelholma (or Kastelholm) castle is from the year 1388 in the contract of Queen Margaret I of Denmark, where a large portion of the inheritance of Bo Jonsson Grip was given to the queen. The heyday of the castle was in the 15th and 16th centuries when it was administrated by Danish and Swedish kings and stewards of the realms. Kastelhoma was expanded and enhanced several times.
In the end of 16th century castle was owned by the previous queen Catherine Jagellon (Stenbock), an enemy of the King of Sweden Eric XIV. King Eric conquered Kastelholma in 1599 and all defending officers were taken to Turku and executed. The castle was damaged under the siege and it took 30 years to renovate it.
In 1634 Åland was joined with the County of Åbo and Björneborg and Kastelholma lost its administrative status.