Leonardo Da Vinci Castle

In some countries, before a castle could be built it was necessary to obtain the permission of the king through a licence to crenellate, or else the builder risked it being slighted – deliberately damaged to such an extent that the castle was undefendable. This was not universal as in some countries the monarch had little control over lords, or required the construction of new castles to aid in securing the land so was unconcerned about granting permission – as was the case in England after 1066 and the Holy Land during the Crusades. Switzerland is an extreme case of there being no state control over who built castles, and as a result there were 4,000 in the country. Before the 12th century, castles were as uncommon in Denmark as they had been in England before the Norman Conquest. The introduction of castles to Denmark was a reaction to attacks from Wendish pirates, and they were usually intended as coastal defences.
• Innovation and scientific design
Until the 12th century, stone-built and earth and timber castles were contemporary, but by the late 12th century the number of castles being built went into decline. This has been partly attributed to the higher cost of stone-built fortifications, and the obsolescence of timber and earthwork sites, which meant it was preferable to build in more durable stone. Although superseded by their stone successors, timber and earthwork castles were by no means useless. This is evidenced by the continual maintenance of timber castles over long periods, sometimes several centuries; Owain Glyndŵr's 11th-century timber castle at Sycharth was still in use by the start of the 15th century, its structure having been maintained for four centuries.
In the late 12th century, there was a change in castle architecture. Until then, castles probably had few towers; a gateway with few defensive features such as arrowslits or a portcullis; a great keep or donjon, usually square and without arrowslits; and the shape would have been dictated by the lay of the land (the result was often irregular or curvilinear structures). The design of castles was not uniform, but these were features that could be found in a typical castle in the mid-12th century. By the end of the 12th century or the early 13th century, a newly constructed castle could be expected to be polygonal in shape, with towers at the corners to provide enfilading fire for the walls. The towers would have protruded from the walls and featured arrowslits on each level to allow archers to target anyone nearing or at the curtain wall. These later castles did not always have a keep, but this may have been because the more complex design of the castle as a whole drove up costs and the keep was sacrificed to save money. The larger towers provided space for habitation to make up for the loss of the donjon. Where keeps did exist, they were no longer square but polygonal or cylindrical. Gateways were more strongly defended, with the entrance to the castle usually between two half-round towers which were connected by a passage above the gateway – although there was great variety in the styles of gateway and entrances – and one or more portcullis. A peculiar feature of Muslim castles in the Iberian Peninsula was the use of detached towers, called Albarrana towers, around the perimeter as can be seen at the Alcazaba of Badajoz. Probably developed in the 12th century, the towers provided flanking fire. They were connected to the castle by removable wooden bridges, so if the towers were captured the rest of the castle was not accessible.