The bishop castle of Haapsalu was built in the 13th century. It was the main residence of the Bishop of Läänemaa. The Läänemaa bishopric was created as a state of the Holy Roman Empire on 1 October 1228.
Construction, widening and reconstruction of the stronghold went on throughout several centuries, with the architecture changing according to the development of weapons. The stronghold achieved its final dimensions under the reign of Bishop Johannes IV Kievel (1515–1527). The western side of the castle houses a 29-metre watchtower dating from the 13th century, later used as a bell tower. The walls were later raised to 15 metres. The inner trenches and blindages, which were built for cannons and as a shelter from bombing, date back to the Livonian War (1558–1582), but it was during this war that the stronghold was severely damaged. The walls of the small castle and the outer fortification were left partly destroyed.
In the 17th century, the castle was no longer used as a defensive building by the Swedes who now ruled the Swedish Estonian Province. In the course of the Great Northern War in 1710, Estonia fell under Russian rule and the walls were partially demolished at the command of the Peter I of Russia, turning the castle in effect into ruins.
The Cathedral of Haapsalu is attached to the castle. It was the official chair of the Bishop, was situated and where the Chapter of the Bishopric worked. It is the biggest single-naved church in the Baltic countries, with its 15.5-metre-high domical vaults and an area of 425 m2. The Dome church was probably built in the 1260s, following the style of the Cistercian Order. The round baptism chapel on the eastern side of the church was built in the 14th or 15th century and is the site of a famous legend. On moonlight August nights the shape of a Lady in White appears on the inside wall of the chapel as the moon shines through the chapel window. This Lady is said to have been a woman who was in love with the abbot and entered the castle against his orders, therefore having been walled in there alive as a punishment. The church has been restored and it is again in active use by the local congregation of the Estonian Lutheran Church since 1990.
Glimmingehus is the best preserved medieval stronghold in Scandinavia. It was built 1499-1506, during an era when Scania formed a vital part of Denmark, and contains many defensive arrangements of the era, such as parapets, false doors and dead-end corridors, 'murder-holes' for pouring boiling pitch over the attackers, moats, drawbridges and various other forms of death traps to surprise trespassers and protect the nobles against peasant uprisings. The lower part of the castle's stone walls are 2.4 meters (94 inches) thick and the upper part 1.8 meters (71 inches).
Construction was started in 1499 by the Danish knight Jens Holgersen Ulfstand and stone-cutter-mason and architect Adam van Düren, a North German master who also worked on Lund Cathedral. Construction was completed in 1506.
Ulfstand was a councillor, nobleman and admiral serving under John I of Denmark and many objects have been uncovered during archeological excavations that demonstrate the extravagant lifestyle of the knight's family at Glimmingehus up until Ulfstand's death in 1523. Some of the most expensive objects for sale in Europe during this period, such as Venetian glass, painted glass from the Rhine district and Spanish ceramics have been found here. Evidence of the family's wealth can also be seen inside the stone fortress, where everyday comforts for the knight's family included hot air channels in the walls and bench seats in the window recesses. Although considered comfortable for its period, it has also been argued that Glimmingehus was an expression of "Knighthood nostalgia" and not considered opulent or progressive enough even to the knight's contemporaries and especially not to later generations of the Scanian nobility. Glimmingehus is thought to have served as a residential castle for only a few generations before being transformed into a storage facility for grain.
An order from Charles XI to the administrators of the Swedish dominion of Scania in 1676 to demolish the castle, in order to ensure that it would not fall into the hands of the Danish king during the Scanian War, could not be executed. A first attempt, in which 20 Scanian farmers were ordered to assist, proved unsuccessful. An additional force of 130 men were sent to Glimmingehus to execute the order in a second attempt. However, before they could carry out the order, a Danish-Dutch naval division arrived in Ystad, and the Swedes had to abandon the demolition attempts. Throughout the 18th century the castle was used as deposit for agricultural produce and in 1924 it was donated to the Swedish state. Today it is administered by the Swedish National Heritage Board.
On site there is a museum, medieval kitchen, shop and restaurant and coffee house. During summer time there are several guided tours daily. In local folklore, the castle is described as haunted by multiple ghosts and the tradition of storytelling inspired by the castle is continued in the summer events at the castle called "Strange stories and terrifying tales".