Vartiokylä hill fort is one of the most prominent historic sites from the age before Helsinki city was established. The fortification dates from the 13th and 14th centuries and it has been in use only for a short period. It is unknown who built it but probably the hill was fortified by Swedish-speaking settlers, crown or some local noble (like Bo Jonsson Grip).
During the first World War Russians built trenches to Vartiokylä hill as part of the Helsinki fortification system. Today you can still see remains of medieval stone foundations.References:
The Petersberg Citadel is one of the largest extant early-modern citadels in Europe and covers the whole north-western part of the Erfurt city centre. It was built after 1665 on Petersberg hill and was in military use until 1963. It dates from a time when Erfurt was ruled by the Electors of Mainz and is a unique example of the European style of fortress construction. Beneath the citadel is an underground maze of passageways that can be visited on guided tours organised by Erfurt Tourist Office.
The citadel was originally built on the site of a medieval Benedictine Monastery and the earliest parts of the complex date from the 12th century. Erfurt has also been ruled by Sweden, Prussia, Napoleon, the German Empire, the Nazis, and post-World War II Soviet occupying forces, and it was part of the German Democratic Republic (East Germany). All of these regimes used Petersberg Citadel and had an influence on its development. The baroque fortress was in military use until 1963. Since German reunification in 1990, the citadel has undergone significant restoration and it is now open to the public as a historic site.