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 stone church of Gamla Uppsala, built over the pagan temple, dates from the early 12th century. Due to fire and renovations, the present church is only a remnant of the original cathedral.
Before the arrival of Christianity in Sweden, Gamla Uppsala was the seat of Swedish kings and a ceremonial site known all over northern Europe. The settlement was home to royal palaces, a royal burial ground, and a great pagan temple. The Uppsala temple, which was described in detail by Adam of Bremen in the 1070s, housed wooden statues of the Norse gods Odin, Thor and Freyr. A golden chain hung across its gables and the inside was richly decorated with gold. The temple had priests, who sacrificed to the gods according to the needs of the people.
The first Christian cathedral was probably built in the 11th century, but finished in the 12th century. The stone building may have been preceded by a wooden church and probably by the large pagan temple.