Vallisaari is an island in Helsinki, Finland. It is located between Suomenlinna and Santahamina. The island has some ponds, and it was first known for hundreds of years as a place where sailors could take fresh water. Some fortifications were built on the island, but while neighbouring Suomenlinna became the main sea fort for Helsinki, Vallisaari was a …Vallisaari is an island in Helsinki, Finland. It is located between Suomenlinna and Santahamina. The island has some ponds, and it was first known for hundreds of years as a place where sailors could take fresh water. Some fortifications were built on the island, but while neighbouring Suomenlinna became the main sea fort for Helsinki, Vallisaari was a supply point where cattle were grazing and firewood was collected.