Hunnic armies relied on their high mobility and "a shrewd sense of when to attack and when to withdraw". An important strategy used by the Huns was a feigned retreat—pretending to flee and then turning and attacking the disordered enemy. This is mentioned by the writers Zosimus and Agathias. They were, however, not always effective in pitched battle, suffering defeat at Toulouse in 439, barely winning at the Battle of the Utus in 447, likely losing or stalemating at the Battle of the Catalaunian Plains in 451, and losing at the Battle of Nedao (454?). Christopher Kelly argues that Attila sought to avoid "as far as possible, [...] large-scale engagement with the Roman army". War and the threat of war were frequently used tools to extort Rome; the Huns often relied on local traitors to avoid losses. Accounts of battles note that the Huns fortified their camps by using portable fences or creating a circle of wagons.