St. Mary's Church (Mariakyrkan) is a fine example of Brick Gothic architecture in Sweden

Sigtuna was founded on what was then the shore of Lake Mälaren just over 1,000 years ago. It took its name from an ancient royal estate (see Uppsala öd) several kilometers to the west (see Fornsigtuna). Various sources claim King Eric the Victorious as founder while others claim King Olof Skötkonung.[3]
It operated as a royal and commercial centre for some 250 years, and was one of the most important cities of Sweden. During a brief period at the end of the 10th and beginning of the 11th century, Sweden's first coins were minted here. St. Mary's Church, built in the 13th century by the Dominican order as a monastery church, still remains largely intact. The Dominican monastery played an important role in the Swedish Middle Ages and produced many important Church officials. Among them, many Swedish archbishops. Many church and monastery ruins still stand, including St. Pers Church (S:t Pers kyrkoruin) dating the 1100s, St. Olof Church (S:t Olofs kyrkoruin) dated from around the middle of the 11th century and St. Lars Church (S:t Lars kyrkoruin) dating from the middle of the 13th century.
In 1187 Sigtuna was attacked and pillaged by raiders from across the Baltic Sea, possibly Karelians, Curonian and/or Estonian (Oeselian) raiders. Archaeological excavations have not verified the traditions of destruction of the town. Normal life in Sigtuna continued until town started to slowly lose its importance during 13th century due to navigability problems caused by post-glacial rebound.
The current coat of arms can be traced to the town's first known seal, dating from 1311. According to a legend (possibly inspired by the town arms) Sigtuna was once the Royal seat, but this can not be confirmed. The crown may also symbolize the large royal mint which was located in the town. Since 1971 the coat of arms has been valid for the much larger Sigtuna Municipality.