You are stopped at night in the centre of Giethoorn, in a narrow, somewhat obstructive spot along the water. What applies here to your sidelights?
Advisable to switch on, but not required — this is inside the built-up area
Mandatory, just like outside the built-up area
Explanation
Wrong — trap: advisable confused with required. Inside the built-up area, sidelights at a dangerous/obstructive spot are advisable, not required — the hard duty only applies outside the built-up area and in a lay-by/on the hard shoulder (Book p.186).