A duty to defend local homesteads
In medieval times in Sweden, all men capable of bearing arms, were obliged to take part in the local defence of their district or region. This form of military service was called folkuppbåd (local militia) and formed the basis for Sweden's armed forces in the Middle ages.
Warring long-ships
For battles beyond local homsteads there were small coastal rading fleets. These were used for war at sea and for plundering raids along the southern and eastern shores of the Baltic.Regions along the Swedish East coast and the Mälar valley counties were divided into rading fleets. Each fleet swore to provide the King with a long-ship or Dragon ship during the warm months of the year. Each ship was manned by an armed crew and provisioned. In early medieval times, fleets of this type had already been in existence for at least a couple of centuries. Much points to the fact that such fleets date back to the 9th Century, in other words, during Viking times.
Fortresses and armoured cavalry
During the 13th Century, the King ceased using fleets of this kind. The reason for this was that he now mainly needed armed forces at home, within the realm. He namely had a problem with Swedes in revolt and invading armies from other countries. The obligation to provide rading fleets was transformed into a tax which was to be paid by those landowners who were reluctant to do military service or who represented the church. Money was needed to build fortresses at strategic locations, to man these fortresses with professional soldiers (mercenaries) and to organise mobile forces with fully equipped cavalry. However, the taxes collected proved barely sufficient to build the foretresses. In order to acquire the armed cavalry he needed, the King granted tax exemption (redemption) to those who placed at his disposal a heavily armed cavalry trooper astride a war horse (the Alsnö decree of 1280). For disputes within the homeland, the King relied on the use of regional militia.
A King at the head of his troops
During the Middle Ages, the King/Regent was not only the Supreme Commander of all armed forces, he also had direct command over all his troops. During the time of the rading fleets, the jarl (an Earl) served as the King's military deputy, whereas in the latter part of the Middle Ages, a field marshal was appointed.
The first cannon
Towards the end of the 14th Century, cannons began to be used to defend Royal castles. Field artillery (cannons hauled on campaigns) were in use in the middle of the 15th Century.