Take the ferry to the Soumenlinna Island from Helsinki Market Square. It is a 15-min trip 2-3 per hour in summer at :00, :20 and :40 past the hour. If you have good weather this is fantastic activity. The views of Helsinki from the sea are superb especially of the Tuomiokirkko white cathedral.
The Island civilian and military inhabitants needed a chruch. It was built on a piece of high ground and the tower was tall. It survived as a lookout platform. Notice the fenced walkway around the top. It also acted as a landmark for passing shipping. A light could be placed in the top of the tower so it acted as a beacon. Old cannons were used as fence posts around the church and they were connected with old iron anchor chain to form a decorative fence.
It is a mariner's church where sailors went to offer thanks for a safe return or pray before going on a new journey. Some of the residents opposite the church have turned their house into a cafe. As you are walking around the island have a close look at the exposed rock surfaces. you will notice that they are scratched all in the same direction. This has been caused by small stones imbedded in the ice at the bottom of glaciers during the last Ice age. As the ice moved over the bedrock it scratched the surface. This physical geographical feature is known as a striation.