One of the images most often associated with the medielval period is that of a castle. Being used both as a residence and a fortress, the castles pretty much defined life during middle-ages Europe. However, building a castle had different consequences for the monarch and the nobles who built them.
If the king/duke/count built a castle on his domain, it served to strengthen his/her rule over that particular land, protecting it and, usually causing a small fair/town to sprout up next to it, improving thus, the economy of the countryside and helping with the tax flow.
On the other hand, castles built and/or expanded by the vassals of this particular king/duke/count signified a loss of control by the latter over the former, as the vassals could claim more autonomy from behind the walls of their fortress, leading to more descentralization of the overlord's realm.
Sieges were also generally more unpleasant for the besieging party rather than for the ones being under siege, and the lord could ill afford wasting his resources in reducing all the castles of his vassals - hence more castles = more independence to the latter. The ruler should, however, have the option of forcing an unruly vassal to reduce his/her own castles under certain circumstances (part of a ransom after being held hostage, part of recongising their succession or as part of a larger pardon, etc.) without necessarely marching his/her troops against them.
If the king/duke/count built a castle on his domain, it served to strengthen his/her rule over that particular land, protecting it and, usually causing a small fair/town to sprout up next to it, improving thus, the economy of the countryside and helping with the tax flow.
On the other hand, castles built and/or expanded by the vassals of this particular king/duke/count signified a loss of control by the latter over the former, as the vassals could claim more autonomy from behind the walls of their fortress, leading to more descentralization of the overlord's realm.
Sieges were also generally more unpleasant for the besieging party rather than for the ones being under siege, and the lord could ill afford wasting his resources in reducing all the castles of his vassals - hence more castles = more independence to the latter. The ruler should, however, have the option of forcing an unruly vassal to reduce his/her own castles under certain circumstances (part of a ransom after being held hostage, part of recongising their succession or as part of a larger pardon, etc.) without necessarely marching his/her troops against them.