Complex sentence: Coenred's reign is poorly documented, but a contemporary source records that he faced attacks from the Welsh, which may later have led Æthelbald to build Wat's Dyke, a defensive earthwork on the northern Welsh frontier.
Split into simpler sentences: Coenred's reign is poorly documented, but a contemporary source records that he faced attacks from the Welsh. The same threat may later have led Æthelbald to build Wat's Dyke, a defensive earthwork on the northern Welsh frontier.