In 1940 when Operation SEALION (invasion of the British mainland) was abandoned, Hitler ordered the building of a night defence system along 1,500 miles of coastline from Denmark to the Spanish border. He was personally involved in the planning which was based round 15 fortresses guarding vital ports. Built by the Todt Organisation using slave labour, the scale of the Atlantic Wall was unprecedented. When the Invasion OVERLORD came the majority of the defences were defended with great vigour and a number held out for longer than expected (Brest and St Malo). Some were by-passed and left in German hands rather than waste lives attempting to capture them.