Orig. a fortified "manor house or private stronghold. Before the Vikings arrived, burh referred to ancient hill camps, and remnants of Roman encampments. Later it was used of fortified towns (which may well have grown up around a fortified manor house); spelling changed to *borough and the word came to signify a municipal district. Alfred was responsible for the establishment of many fortified burhs, new towns in effect, as part of a strategy of defence against the Vikings. In Wessex, few places were further than 20 miles from a burh. Many placenames ending in '-bury' might well have been burhs. Often such places thrived because of their markets. However, *burhbot applied only to walled towns and the fortified places established by Alfred. In the sense of a manor house, possession of a burh marked the graduation from *ceorl to *thegn.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.