A former island fortress, Suomenlinna is a perennially popular collection of museums, parks, and gardens, and has been designated a UNESCO World Heritage Site. In 1748 the Finnish army helped build the impregnable fortress, long referred to as the Gibraltar of the North; since then it has expanded into a series of interlinked islands. Although Suomenlinna has never been taken by assault, its occupants surrendered once to the Russians in 1808 and came under fire from British ships in 1855 during the Crimean War. Today Suomenlinna makes a lovely excursion from Helsinki, particularly in early summer when the island is engulfed in a mauve-and-purple mist of lilacs, introduced from Versailles by the Finnish architect Ehrensvärd.
Suomenlinna is easily reached by public ferry (€2.50 one-way, €3.60 round-trip) or private boat (€3.50 one-way, €5.50 round-trip), both of which leave from Helsinki's Kauppatori. The ferry ride from South Harbor to Suomenlinna takes about 15 minutes.
Plan to spend an afternoon on the islands; you'll need about four hours to explore the fortress and museums. Note that days open and hours of sites are limited off-season.
Although its fortification occupied six islands, its main attractions are now concentrated on three: Iso Mustasaari, Susisaari, and Kustaanmiekka. There are no street names on the island, so get a map for about €2 from the Helsinki City Tourist Office before you go, or buy one at the visitor center on the island.
From June through August, guided English-language tours leave daily at 11 am and 2 pm from the Suomenlinna Visitor Centre; call to arrange tours at other times. The center, which is in the same building as the Suomenlinna Museum, is on the shore of Tykistölahti Bay, about 400 yards south of the main ferry terminal