The following description from the Wikipedia page on Cassel succinctly describes the events during the fighting in and around Cassel during the retreat to Dunkirk.

The 2nd Battalion, Gloucestershire Regiment and the 4th Oxfordshire and Buckinghamshire Light Infantry held Cassel for three days as part of the defensive screen around Dunkirk during the Battle of Dunkirk and evacuation (27 May 1940 – 30 May 1940). The British forces had prepared a defence on the hilltop, emplacing anti-tank guns and barricading the narrow streets of the town. After scoring initial successes against the tanks of Panzer Regiment 11, which had made the mistake of advancing without infantry support, the British garrison was heavily attacked from the ground and the air by German forces. Much of the town was reduced to ruins by bombing. Most of the garrison’s members were killed or captured by the Germans during the fighting or the subsequent attempted breakout towards Dunkirk, but the defence they had put up played an important role in holding up the Germans while the Dunkirk evacuation was taking place  

The photos show the very picturesque town taken in 2012 which include a heavily scared building, a plaque to the Gloucesters and the Ox and Bucks and finally views of the hill showing the commanding position of the town against the surrounding countryside.