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 ancient Argos Theater was built in 320 BC. and is located in Argos, Greece against Larissa Hill. Nearby from this site is Agora, Roman Odeon, and the Baths of Argos. The theater is one of the largest architectural developments in Greece and was renovated in ca 120 AD.
The Hellenistic theater at Argos is cut into the hillside of the Larisa, with 90 steps up a steep incline, forming a narrow rectilinear cavea. Among the largest theaters in Greece, it held about 20,000 spectators and is divided by two landings into three horizontal sections. Staircases further divide the cavea into four cunei, corresponding to the tribes of Argos A high wall was erected to prevent unauthorized access into the theatron and may have helped the acoustics, but it is said the sound quality is still very good today.
Around 120 CE, both theaters were renovated in the Roman style.