For my visit to the Defense Line, I chose Muiden and Pampus, two of the most accessible locations. Muiden is a historical fortress-town to the east of Amsterdam. The whole town is like an open-air museum and has a real historical feel to it. The Muiden Castle is also a must.
From Muiden I took the ferry to Pampus. This island became the final part of the Defense Line: it had to guard the entrance to Amsterdam via the IJ-river. Soldiers were trained here between the two World Wars, but never has the island been used in combat. The place was already outdated when its construction had finished.
Nowadays Pampus sees 35.000 visitors a year. Since 1990 it has been privately owned by a foundation that wants to maintain and restore the fortress island. They haven't been lucky in finding funds though, and so the site is in really bad repair. Not as bad however as when they acquired it twelve years ago when there were rats everywhere and trees grew out of the buildings.