Kronstadt sits on Kotlin Island in the easternmost reach of the Gulf of Finland, twenty-five kilometers due west of central Saint Petersburg. Peter the Great had picked the spot in 1703 — the same year he founded his new capital — because it was the only place where deep-water shipping could be channeled past defenses on its way to the imperial city. By 1790 the island had become one of the most heavily fortified naval bases in Europe, ringed with batteries on artificial stone islands set into the shallow water. The Russian Kronstadt squadron under Admiral Aleksandr Kruse had a defensive job, not an offensive one. Kruse did not need to defeat the Swedes. He only needed to delay them.

Duke Charles's fleet met Kruse's squadron on the morning of 3 June 1790. The two forces were roughly equal in strength. They formed lines of battle and exchanged fire for four hours without either side gaining a clear advantage. The Swedish coastal flotilla, which had been clawing its way east through the gulf in support, sortied to help — but the small archipelago vessels could not keep pace with the great ships of the line, and by the time they arrived the day's fighting was over. They turned back to the Beryozovye Islands. The next day, 4 June, the fleets clashed again with even less to show for it. Kruse, an experienced commander, had no intention of risking his squadron in a decisive engagement. He needed only to keep Duke Charles occupied.

What Duke Charles had not yet realized was that Admiral Vasily Chichagov was sailing east from Reval with the main Russian battlefleet — twenty-nine ships of the line, eleven frigates, and an array of smaller craft. By 6 June, the two Russian forces had linked up. Now significantly outnumbered, the Swedish naval commanders wanted to retire west to Sveaborg, where battle damage could be properly repaired. Gustav III refused. The king insisted that the open-sea fleet stay close to the coastal flotilla, which meant anchoring at the head of the Bay of Vyborg for repairs. By 26 June, Chichagov's ships had quietly worked their way to within two nautical miles of the Swedish anchorage — closing a ring around both Swedish fleets, the king himself, his brother, and roughly thirty thousand men. The Battle of Vyborg Bay, which followed on 3 July when the Swedes fought their way out, would be one of the largest naval battles in Baltic history and would cost Sweden seven ships of the line and several thousand sailors.

Considered on its own, the Battle of Kronstadt was a draw — a few days of cannonade, no ships sunk, casualties light by the standards of the war. Considered as part of the campaign, it was a Russian strategic victory. Kruse had done exactly what he was asked to do: delay the Swedes long enough for the rest of the Russian Baltic Fleet to come up. Some historians have argued that Gustav III's decision to push east was nevertheless inspired — for those few days in early June 1790, with most Russian ground forces deployed elsewhere, the Swedish fleet was at the doorstep of Saint Petersburg at a moment when the city had almost nothing on land to oppose them. Whether anything could have come of that opportunity is impossible to know, because the Swedish fleet did not have the strength or the supplies to exploit it. The war ended two months later in a peace treaty that restored the prewar borders and changed nothing — except that Gustav III, returning home to claim a king's vindication, would be assassinated at a masquerade ball in Stockholm less than two years later.

The fortress island that the Swedes never reached would dominate Russian naval history for two more centuries. Kronstadt was the home base of the Russian Baltic Fleet through the wars against Napoleon, the Crimean War, and the World Wars. In 1921, in one of the most painful episodes in Soviet history, the sailors who had once been the vanguard of the Bolshevik revolution rose against the new government and were crushed by the Red Army across the same frozen ice the Swedish ships had once sailed past. Today the island sits at the western end of the Saint Petersburg Flood Prevention Facility complex, the great seawall that protects the modern city from Baltic storm surges — the same flat, shallow approach that Admiral Kruse defended for two days in June 1790.