Svartholm fortress The Svartholm fortress ( ; ) was built between 1749 and 1764 outside Loviisa in Southern Finland by Augustin Ehrensvärd . The fortress , which lies at the mouth of the Bay of Loviisa , along with the planned land fortress at Loviisa , would prevent invading Russian forces to enter Swedish domain in present - day Finland . Construction . After Swedish defeats in the Great Northern War as well as in the Russo - Swedish War of 1741 - 1743 there was pressing need to construct fortifications both to the border as well as to the coast of Finland . Already in 1745 Degerby ( later Lovisa - ) was deemed to be suitable place for border fortification and to protect it from the hostile naval forces a seafortress of Svartholm was required to be built . Main base of operations and a base for Swedish naval forces would be built to Sveaborg ( ) . Plans for the fortifications were prepared by Lieutenant - Colonel Augustin Ehrensvärd who was also tasked with constructing the fortresses in 1747 . Constructions started on 1748 and continued with increasing tempo as the foreign political situation was deteriorating in 1749 and 1750 . Over half of the Swedish army in Finland was tasked with the construction and had in 1750 altogether over 6 000 men working on fortifications at Svartholm and Sveaborg . Ehrensvärd had tasked first Captain O . R . Clansenstierna and later ( 1751 - 1757 ) Lieutenant - Colonel Fabian Casimir Wrede with the construction efforts at Degerby and Svartholm who had in 1751 around 2 000 workers at their disposal . While the plans for the fortifications around the town had to be cut short it did not hinder the construction of Svartholm . King Adolf Frederick visited the site in 1751 which was renamed from Degerby to Lovisa ( ) after his wife Louisa Ulrika during his reign . Like at Sveaborg the Swedish participation to the Seven Years ' War brought construction effort to a standstill . Work on the fortifications continued in 1770s and after Ehrensvärd died were led in 1773 - 1774 by Jacob Magnus Sprengtporten but after his break with the King Gustav III works slowed down again . In 1775 works at fortifications around Lovisa were stopped when of the six planned bastion s , only two were ready and efforts at Svartholm slowed further down and in 1778 only some minor construction took place . In 1788 when new war against Russia started the main fort of Svartholm was completed but the breastworks were not . Swedish service . While Svartholm had been constructed as a seafortress it had never been designed to act as a naval base and had only its guns to offer as a support for the naval forces . Order to prepare the fort was issued already on 22 May 1788 . While Svartholm did not really see any action in the Russo - Swedish War of 1788 – 1790 it did act as important staging ground and rallying point during several stages of the war . The fortress at Svartholm was still not ready in 1808 , when the Finnish War erupted , and two third of the guns and mortars were available . It had however been noted already in the 1760s , that it was not possible to place as many guns on the fortress as would be required to repulse a big attack from the sea . It had also been noted that the southern part left much to wish for . It was armed with 86 guns and 8 mortars and gad garrison of 700 men . However the fort had fallen into neglect with most of gun carriages having rotted away while others had not even had any carriages to begin with . Small arms were in similar condition and fort lacked both food and ammunition which had not been stocked in sufficient quantities . Commander of Svartholm before the war had been Captain Carl Gustaf von Schoultz who had to tend with badly trained garrison of which only third could be armed with a functional weapon . Garrison had low morale and bad discipline which was not improved by the sudden change of commander as Major Carl Magnus Gripenberg assumed command on 15 February . However Gripenberg was not highly appreciated by his superiors or his subordinates . Effort was however made to prepare the fort for the war but war started long before work on the fortress would have been completed . Ammunition was in short supply especially for 12 and 18 pounder guns . There was also shortage of food which was further complicated by the fact that the drinking water was also limited as several of the wells had been found unusable . Lovisa was the first goal of the Russians in the Finnish War ( 1808 – 1809 ) , and the Russian main force crossed the border at Abborrfors on February 21 , 1808 . The Russians quickly encircled Svartholm already on 22 February 1808 and demanded surrender which was refused by the Swedes . On 23 February additional Russian forces were brought in to besiege the fort bringing Russian strength to around 1 700 men . However messengers could still reach the fort and it took until 28 February before Russian cossack patrols cut the final routes to the fort . Russians attempted first on 2 March and on 8 March to get the fortress to surrender by negotiations but Swedes refused . When Russian artillery started bombarding the fort it soon became apparent that only 8 of the whole forts guns could be brought to shoot at the Russians as the narrow firing slits made it impossible to use the rest of the guns . Fort was already suffering from the siege with one sixth of the garrison sick and having shortage of warm clothing and food . Morale crept even lower and officers feared mutiny . On 11 March Gripenberg suggested truce of six weeks which the Russians Major General Muchanoff promptly rejected but agreed for a truce of three days after which hostilities would continue anew . Gripenberg agreed to a surrender on 14 March before the truce had run out . On 17 March Russian General commanding the armies in Finland , Friedrich Wilhelm von Buxhoeveden , accepted the surrender . Swedes surrendered the fortress with all its weapons intact to the Russians who in turn agreed just to dismiss the mostly Finnish garrison of Svartholm . The reasons for the capitulation are somewhat unclear but it seems like the Swedish officers didn ' t believe in the Swedish capability , nor the fortress ' capability to withstand the Russians in this war . As many of the other Swedish officers , Gripenberg entered Russian service after the capitulation . He was labeled a traitor in Sweden and was sentenced to death , along with other officers , for the loss of Finland . Due to a general amnesty , the death penalty process was interrupted , also for him . Russian service . Svartholma lost its strategical importance during the Russian period . It was used partly as a military base , and partly as a prison for Finnish prisoners . The fort was largely destroyed by the British during the Crimean War on July 7 , 1855 , but large parts survived the war . It continued to serve as a good harbor and a goal for weekend picnics . The Finnish National Board of Antiquities were restoring the castle since the 1960s , and the work was finally ready in 1998 .