The largest and best-preserved colonial fort on Java, Fort Willem I — known locally as Benteng Pendem, the Sunken Fort, because of its half-buried construction — is an impressive complex of red-brick barracks, walls and inner courtyards. Walking among the empty wings is genuinely atmospheric: trees pushing through floors, plaster falling in sheets, swallows nesting in the high windows.
Only the disused portions are open to visitors; the rest remains a working penal facility. Entry is informal — a small fee to the gatekeeper. Combine with the Train Museum (ten minutes away) for a half-day of colonial-era heritage.