Charles invaded Norway in 1716 with a combined force of 7,000 men. He occupied the capital of Christiania, (modern Oslo), and laid siege to the Akershus fortress there. Due to a lack of heavy siege cannons he was unable to dislodge the Norwegian forces inside. After suffering significant losses of men and materiel, Charles was forced to retreat from the capital on 29 April. In the following mid-May, Charles invaded again, this time striking the border town of Fredrikshald, now Halden, in an attempt to capture the fortress of Fredriksten. The attacking Swedes came under heavy cannon fire from the fortress and were forced to withdraw when the Norwegians set the town of Fredrikshald on fire. Swedish casualties in Fredrikshald were estimated at 500 men. While the siege at Fredrikshald was underway, the Swedish supply fleet was attacked and defeated by Tordenskjold in the Battle of Dynekilen.