An exact nature of the walls of a medieval town or city would depend on the resources available for building them, the nature of the terrain, and the perceived threat. In northern Europe, early in the period, walls were likely to have been constructed of wood and proofed against small forces. Especially where stone was readily available for building, the wood will have been replaced by stone to a higher or lower standard of security. This would have been the pattern of events in the Five Boroughs of the Danelaw in England.