According to some early medieval sources, the Siege of Exeter or Siege of Caer-Uisc was a military conflict that took place in or around 630 CE, between the Mercians, led by Penda of Mercia, and the Britons occupying Caer-Uisc (Exeter) in the kingdom of Dumnonia. Penda is said to have laid siege to the town until the exiled British High King Cadwallon of Gwynedd, arrived to confront him. An alliance between British and Mercian forces followed, secured by Cadwallon's marriage to Alcfrith, Penda's sister, and they marched north to face the armies of Northumbria (who were occpying Gwynedd) at the Battle of Cefn Digoll.

• http://en.wikipedia.org/wiki/Siege of Exeter (c. 630)