To have a successful defensive line, be prepared to defend it. In this case, there are 20 tanks in Coventry ready to close any breaches, as well as plenty of artillery fortified on the mountain in the northwest.

The historical Maginot Line defended the French border with Germany, but did not protect the border with Belgium. The German battle plan, Sichelschnitt, or "cut of the sickle," bypassed the Maginot Line, and subjected France to a swift and humiliating defeat. The reasons for the Maginot debacle are many. Here is a good article on the subject.