Most towns of any size have a PMU, Cassel is lucky to have the family run 'A Sainte Cecile'.
There is a loyal group of regulars, supplemented by tourists and day trippers. The owners know about wine, so expect something a little special compared to usual café fare. Then there are the beers, normally Baston on tap from La Brasserie du Goulot in Wimereux, and often something from the local 3 Monts. Then there is Queue de Charrue, and Maes pils from Belgium, plus a good selection of bottled beers, including Chimay and Amosteké.
There is a large terrace out front, with a beautiful view down through Rue d'Aire to the plain below.
Food is simple croques, or sometimes planche of carcuterie, and you are welcome to 'pic-nic' with frites or sandwiches from the local outlets.