A levy or tax raised specifically for the repair or upkeep of a *burh's fortifications. It is first mentioned in a charter of King Aethelbald in 749. It could also require men to work, as happened during construction of Offa's Dyke. It applied particularly to those fortified places established by Alfred as part of the general defences set up against Vikings.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.