We visited Suomenlinna last Friday and did the guided tour in English from the museum. It cost 15 euro, took a bit more than the official hour, and was very entertaining and informative. While our (current) intro says that Suomenlinna was built "to guard the entrance to Helsinki's harbour", according to our guide, that wasn't its purpose. Helsinki was of no great importance back then and only gained in status after the fortress was built.

The archipelago that houses Suomenlinna (Sveaborg) is en route from St. Petersburg to Stockholm. When it became known that the tsar was building a fleet of ships that could sail along the coast with its many islands, Sweden felt threatened. They chose to build their own (secret) shipyard and Suomenlinna was to be the location. So it was actually a shipyard for relatively small naval ships. The fortress was defending the dock and hiding that there was a shipyard. And it was in the defence of Stockholm itself, more than Helsinki. (See about the dry dock, e.g. here: https://suomenlinna.fi/en/whs-suomenlinna/)

The dry dock is still there (photo), but has no military use anymore. But it is functional (with a changed entrance and pumping system) and is now used for private ships/boats. Tourists aren't supposed to go down into the dock, but there is an observation platform that provides a great view.

It may also be good to mention that the ferry ticket is a regular bus/tram ticket (single fare 3.40 euro, I think) and especially if you are already using public transport in the city, just buy a 10-euro day ticket for the city and you can step on the ferry without extra charge. The tight public transport network makes it very easy to get around in Helsinki.