Lilleborg is a ruined castle in the Almindingen forest. The castle was probably built in the middle of the 12th century as a royal fortress. It appears to have replaced the much larger fortress of Gamleborg which was only 700 metres away. The move could be explained by the fact that in 1149, three-fourths of Bornholm had been surrendered to Eskil, archbishop of Lund. As a result, King Sweyn III wanted to establish his own seat of power on the remaining fourth of the island. While it could not be compared to the archbishop's Hammershus, Lilleborg was nevertheless a fine stronghold and appears to have been easier to defend than the much larger Gamleborg. There is evidence suggesting the fortress burnt down in 1259 when Prince Jaromar of Rügen stormed and destroyed the 'king's stronghold' on Bornholm. However, coins minted after that date have been found, indicating it was inhabited after its storming.
Lilleborg's construction inland rather than on the coast has been compared to that of Refshaleborg on Borgø in Maribo lake. It had a large tower, 9.5 metres square, protecting the entrance to the courtyard. Its walls were 2.4 metres thick and from the top it was easy to hit anyone trying to attack. The fortress is further protected by a curtain wall which follows the cliff tops around the oval plateau. The interior was some 76 metres long and 41 metres wide. When its defenses were obsolete, the fort's stones were re-purposed for other buildings. From 2006 to 2011, restoration work was carried out at Lilleborg.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.