In January 1322, Mortimer attacked and burnt Bridgnorth but, being heavily outnumbered, was forced to surrender to the king at Shrewsbury. Mortimer joined Lancaster at the Battle of Boroughbridge in March 1322 and warrants for his arrest were issued in July. A death sentence was passed upon Mortimer but this was commuted to life imprisonment and he was consigned to the Tower of London. In August 1323 Mortimer, aided by Gerald de Alspaye, the sub-lieutenant or valet of the Tower's Constable, drugged the warders during a feast, allowing Mortimer to escape. He attempted to capture Windsor and Wallingford Castles to free imprisoned Contrariants. Mortimer eventually fled to France, pursued by warrants for his capture dead or alive.