The other thing is that in 1914/15 only the Germans really had a sizeable modern siege artillery (okay, they nevertheless asked the Austrians to lend them some Skodas). - Trying to defend Calais as a kind of bridgehead would be no good idea in the face of the German heavy artillerie - even leaving out the Fat Berthas and 30.5 cm Küstenmörser, the 210 mm howitzers were able to destroy everything not buried deep and protected by several inches of concrete.