Although a number of the forts were closed I did manage to visit a few of them:

Aalsmeer fort - got a guided tour from the curator of the museum here, saw the facilities provided for the troops stationed here virtually undergound and enclosed in a massive reinforced concrete structure (train to Schiphol Airport then bus #198 to Aalsmeerdenbrug).

Fort Uitermeer - a circular fort (train to Weesp then bus #106 to Rondelvrugt.

Vesting Weesp - a circular fort (train to Weesp)

Muiden Slot - has more an appearance of a stately home or castle than a defence installation (by bus from Weesp).

Pampus Island - the ferry ride from Muiden included an extensive guided tour of the fort.

Amstelhoek fort - by bus #142 from Amsterdam Centraal to Tienboerenweg, Uithoorn.

Fort an der Drecht - now a restaurant, winery and art studio (bus #170 from Uithoorn).

Fort Vijfhuizen - now an art centre (by bus #140 from Uithoorn to Hoofddorp then bus #300).

Fort Veldhuis and Fort an der Ham - found these on a hike along the dykes from Heemskerk (bus #167 from Beverwijk station) to Krommenie-Assendelft station.