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Suomenlinna, the ‘fortress of Finland’, straddles a cluster of car-free islands connected by bridges. The Unesco World Heritage site was originally built by the Swedes as Sveaborg in the mid-18th century. Several museums, former bunkers and fortress walls, as well as Finland's only remaining WWII submarine, are fascinating to explore; its tourist office has info. Cafes and picnic spots are plentiful.

Ferries (www.hsl.fi; one-way/return €3.20/5, 15 minutes, four hourly, fewer in winter) depart from the passenger quay at Helsinki’s kauppatori.

From May to September, JT-Line runs a waterbus from the kauppatori, making three stops on Suomenlinna (20 minutes).

At Suomenlinna's main quay, the pink Rantakasarmi (Jetty Barracks) building is one of the best preserved of the Russian era. It holds a small exhibition and the helpful, multilingual tourist office, with downloadable content for your smartphone. Guided tours of Suomenlinna depart from here.

Near the tourist office you'll find a hostel, a supermarket and Suomenlinna's distinctive church. Built by the Russians in 1854, it served as a Russian Orthodox place of worship until the 1920s when it became Lutheran. It doubles as a lighthouse – the beacon was originally gaslight, but is now electric and still in use.

From the main quay, a blue-signposted walking path connects the key attractions. By the bridge that links Iso Mustasaari and the main island, Susisaari, is Suomenlinna-Museo, a two-level museum covering the history of the fortress.

The most atmospheric part of Suomenlinna, Kustaanmiekka, is at the end of the blue trail. Exploring the old bunkers, crumbling fortress walls and cannons will give you an insight into this fortress, and there are plenty of grassy picnic spots. Monumental King’s Gate was built in 1753–54 as a two-storey fortress wall, which had a double drawbridge and a stairway added. In summer you can get a water bus back to Helsinki from here, saving you the walk back to the main quay.

Several other museums dot the islands, including the absorbing Ehrensvärd-Museo, once the home of Augustin Ehrensvärd, who designed the fortress. Outside, Ehrensvärd’s elaborately martial tomb sits opposite Viaporin Telakka, a picturesque shipyard where sailmakers and other workers have been building ships since the 1750s. The dry dock holds up to two dozen boats; these days it's used for the maintenance of wooden vessels.

Along the shore from here is another fish out of water. The Vesikko is the only WWII-era submarine remaining in Finland. It saw action against the Russians. It’s fascinating to climb inside and see how it all worked. Needless to say, there’s not much room to move.

Back on Iso Mustasaari is Sotamuseo Maneesi, which has a comprehensive overview of Finnish military hardware from bronze cannons to WWII artillery. Quite a contrast is the nearby Lelumuseo, a delightful private collection of hundreds of dolls and almost as many teddy bears, which has an on-site cafe.

There are several other places to eat and drink; the best for either is Suomenlinnan Panimo, by the main quay. It brews a clutch of excellent beers, including a hefty porter, and offers good food to accompany it. Taking a picnic is a great way to make the most of Suomenlinna’s grass, views and (hopefully) sunshine. At around 5.15pm it’s worth finding a spot to watch the enormous Baltic ferries pass through the narrow gap between islands.