The sheer breadth and depth of types and sizes of fortification extant in the 8th Century is so extensive that it's very hard to give any form of comprehensive answer to this question, I'm afraid. I would recommend Robert Bartlett's The Making of Europe, Steven Bassett's work on the early medieval West Midlands and Hajnalka Herold's work on fortified settlements in Eastern Europe to give you an idea of the more typical varieties of sites.

Although the Alfredian network of burh sites would not appear in England for over a century, a network of fortifications was maintained in Offa's 8th Century Mercia. At some sites, like Worcester or Tamworth, the defences took the form of one or two earthen ramparts, surrounded by a wide, deep ditch and topped with a wooden palisade with a fighting step. Stone buildings were few and far between, limited largely to churches and perhaps some palaces, but the majority of buildings would have been largely of timber construction in most areas.

In contrast, many Roman settlement defences also survived in England. At settlements like Rochester, Rocester, Canterbury and Exeter, the Roman stone walls still stood, and stone building was more prevalent. In other settlements like London and Chester, the Roman walls were still extant, but would not be repaired until the 9th or 10th centuries.