Ekolsund Castle was once a royal summer residence, most closely associated with Gustav III. Today the interiors are not the main reason to come; the value is the parkland, arboretum, and the sense of old aristocratic Uppland just outside the commuter belt.
Access can be seasonal and tour-based, so check before setting out. If it is open, it gives Enköping's countryside circuit a stately counterweight to the older rock carvings at Boglösa.