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 Odeon of Herodes Atticus is a stone theatre structure located on the southwest slope of the Acropolis of Athens. It was built in 161 AD by the Athenian magnate Herodes Atticus in memory of his wife, Aspasia Annia Regilla. It was originally a steep-sloped theater with a three-story stone front wall and a wooden roof made of expensive cedar of Lebanon timber. It was used as a venue for music concerts with a capacity of 5,000. It lasted intact until it was destroyed and left in ruins by the Heruli in 267 AD.
The audience stands and the orchestra (stage) were restored using Pentelic marble in the 1950s. Since then it has been the main venue of the Athens Festival, which runs from May through October each year, featuring a variety of acclaimed Greek as well as International performances.