Braathen Dendrokronologiska Undersökningar
Citadels 'Kastaler' on Gotland
'Kastalen', the tower at Gammelgarn and other similar towers
On Gotland there are remains of ten stone buildings from the Middle Ages, which in written sources from the 17th and 18th centuries are called 'kastaler', (sing.: kastal, plural: kastaler). The word generally is translated into english as citadel with rather vague definition. The word 'kastal' still is used.
Wood samples from five of these towers have been investigated, but only samples from the tower at Gammelgarn have been dated.
A reason for mentioning the group of towers is that two of them contain pieces of wood which are difficult to reach but can be investigated after much effort. An other reason is that when putting the attention to the tower at Gammelgarn it is an advantage for the reader to have at least some piece of information on the detached towers near churches, e.g. the construction of the walls.
The reason why these stone buildings were erected less than 100 m from churches is not known, partly because archaeological excavations have only started. The remains of the buildings are situated in order according to the following, we start at Visby and go south to Västergarn and Fröjel and turn back at Sundre, then we pass Hamra, go north to Alva, turn to the eastern side of the island and find Ardre, Gammelgarn, Kräklingbo and Gothem. At the northern part of the island is Lärbro.
At Fröjel, Sundre, Gammelgarn and Gothem there are ruins with upright standing stone walls from towers, while at Lärbro most of the tower remains unchanged from the erection period. At the rest of the places mentioned there are stone remains in the ground showing the base of a building with about quadratic or circular shape and with an outer area between about 40 and 90 square-meters. The bases at Västergarn and Sundre are nearly circular.
The upright standing ruins bear traces of the same standard of professional knowledge in building construction as one can find in the medieval stone houses of Visby and in the churches. The individual variations are much accentuated as is the case in the construction of churches.
The ten buildings and sites mentioned have in literature and in common views been considered as some kind of local defence, and because of their geographical distribution they are said to have been part of a defence system on the island. I cannot see how these buildings could be efficient as part of the defence. If I were an enemy I would be pleased if I could keep local soldiers shut up in the towers and I would just pass out into the district to do my mission with better success than if all the local soldiers were free to move around.
A fortification is only built in order to prevent the enemy from passing an area or reaching something which is of much value, e.g. a town. Just to attack a fortress normally means heavy losses for the intruders and must be balanced against what there might be to win. Within the defence it is of importance to move the concentration of arms without delay to the spots where they temporarily are needed. Because of that the action leading staff must have a position from where they can get instantaneous information on what is going on and from where their orders safely and with swiftness can be conducted to the troops. For that reason in the middle age a tower was built with central position within the fortress. It was called a keep or a donjon. Even if the enemy succeeded in making a break through it was of most importance to analyze the situation and transmit orders safely. For that reason it was also possible to give efficient resistance from the keep and maintain an efficient leadership in the battle.
One example near to hand is the tower Kruttornet close to the northern part of the harbour of Visby. A similar tower was situated close to the quarter Residenset near the southern part of the harbour. Both towers required some kind of defence obstacles in the surrounding area if they were built as keeps.
Those towers which were built less than 100 m from a number of parish churches on Gotland lack the function of a keep. They were much suitable as store-houses preserving merchandise with low risk of fire and burglary. It is known that the church in the middle age collected tax in form of products from agriculture and did trade in a league of their own and in cooperation with established merchant societies.
If we assume that the towers were built as store-houses some elements of construction will easily be given a different interpretation of function. Those holes in the walls which have been said to be latrine-pipes, were useful as holes through which it was possible to unload corn which could flow in wooden chutes down to bags or sacks. The door-ways in the second storeys have been suitable for unloading goods. To preserve corn during the winter season requires a dry surrounding area where it is easy to keep rodents away. Corn which is preserved in sacks for a long time will be infested by mould and then is not suitable for consumption. For that reason it is preserved in bins. The towers are suitable as granaries. The towers at Fröjel, Gammelgarn and Gothem stand on dry ground while the tower at Lärbro stands on moist ground with moisture under the floor in the barrel vaulted ground storey. Moist air has lower density than dry air at the same temperature and has power to float upwards. For that reason the vault in the ground storey is a good hindrance against air floating up into the higher storeys.
It is however probable that there has been some other concealed reason for planning a vault in the ground storey.
In the towers of Fröjel, Gammelgarn and Gothem the relations between thickness of the walls in different levels are difficult to explain. The dimensions probably have been decided already at a planning stage. It is known that among the different servants of the church has been a strong state of obedience to the superiors. Let us imagine that the builder was set by superiors to build according to an existing drawing, designed for a church tower, and showed more consideration for the instructions of the drawing than for the practical function of the planned building and the costs of erection.
Literature: Ola Kyhlberg, Gotland mellan Arkeologi och Historia. Stockholm 1991