About 20km south of Lenzburg is Wasserschloss Hallwyl, a ‘Water Castle’ so named because it's built in the middle of a river – a natural moat. It is, in fact, two modest castles joined by a bridge. Walking paths wind southwards towards Hallwilersee, a peaceful lake that can be criss-crossed by ferry.
From Aarau, take a regional train to Lenzburg then change to a regional train to Boniswil (Sf11.60, 12 minutes), from where it’s a 1km walk east to the castle (follow signs for Seengen).