Iso Linnamäki: A Medieval Fortified Hill in Porvoo, Finland
Visitor Information
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Official Website: www.luontoon.fi
Country: Finland
Civilization: Unclassified
Remains: Military
History
Iso Linnamäki is a fortified hill located in the municipality of Porvoo, Finland. The site was originally established during the Middle Ages, built by local medieval Finnish communities whose activities reflect both defensive needs and settlement patterns along the Porvoonjoki river.
Archaeological evidence shows that human presence at Iso Linnamäki dates back to the Iron Age around the 7th to 9th centuries, as indicated by layers of burnt soil found on the hill. This early activity likely connected the site to a nearby settlement and the Pikku Linnamäki cemetery, which was once an island in the estuary of the Porvoonjoki but eventually joined the mainland. These indications suggest that Iso Linnamäki’s location held strategic and possibly ritual importance long before medieval fortifications appeared.
The main fortification phase at Iso Linnamäki is generally assigned to the late 14th century. While precise records are lacking, it is possible that local noble figures such as Bo Joninpoika Grip or Jeppe Djäkn were responsible for commissioning the defensive works. The site may correspond to a medieval place called Wartholm referenced in historical documents. Although there is some speculation connecting the site to Danish crusades in Uusimaa in the late 12th and early 13th centuries, no direct evidence has confirmed this link. The earliest written record mentioning the hill appears in a complaint list from 1555 to 1556 compiled by Jakob Teitin, affirming the site’s recognition in post-medieval times.
Usage of the fortress was relatively brief and likely lasted only a few decades. Over time, the southern bay known as Maarin satama (Maarin harbor), once situated just south of Iso Linnamäki, gradually silted up, altering the landscape. Later modifications to the fortifications happened in the early 20th century when the dry moats were deepened, and sand was extracted from the southern slope of the hill. After the medieval period, evidence such as coins and firearm parts shows that the area maintained some military function.
Today, Iso Linnamäki is regarded as part of a nationally important cultural heritage area together with Old Porvoo, reflecting its continuing significance through the centuries.
Remains
Iso Linnamäki sits atop a moraine ridge that rises nearly 30 meters above sea level near the Porvoonjoki river. The hill’s summit comprises a roughly rectangular plateau measuring approximately 32 by 47 meters. This central area was enclosed by two dry moats—trench-like earthworks dug around the plateau to act as defensive barriers. Constructing these moats required considerable effort, involving the movement of large amounts of soil to form protective ditches. The existing moats were deepened in the early 1900s, slightly altering their original medieval profile.
No stone defensive structures have been found on the ramparts, suggesting that any fortifications such as walls, palisades, or buildings were made of wood and have since disappeared. On the plateau itself, two large pits remain, each about 10 meters wide and 2 meters deep. These are interpreted as storage cellars, probably used for preserving food or supplies.
Excavations have revealed further features connected to daily life and military use. Among the finds are stone hearths for heating or cooking and iron tools, including crossbow bolt heads dating from the 13th to 14th centuries. A number of these bolts suggest that the site was actively defended or engaged in armed conflicts during the medieval period. Fragments of a German stoneware jug from the mid-1300s indicate trade contacts or the presence of imported goods.
Historically, the plateau also contained essential buildings such as a manor house known as a linnantupa, a kitchen, and a smithy, framing Iso Linnamäki not only as a military stronghold but also a self-sufficient estate. The southern slope of the hill has been altered by sand extraction work, which modified the medieval terrain.
Surrounding the site today is a pine forest, while the area between the former harbor and the hill supports various plant species introduced relatively recently to Finland’s natural environment. These include medicinal and spice plants like wormwood, greater knapweed, and dark mullein, hinting at the site’s long history of human influence on the landscape.
Iso Linnamäki remains preserved as a protected area, with modern pathways and signage that mark its historical significance and aid in understanding the castle hill’s enduring place in Porvoo’s heritage.
