The 87m-tall Unesco-listed baroque belfry is a 17th-century marvel topped by black-and-gold mini-domes. The lift takes you up to only 36m, though the panoramic views are still impressive (you can climb a further two flights of stairs for a look at the clock mechanism and bells). The surrounding garden (admission free) occupies the hilltop site of what was originally Mons’ fortress.
Note that the entrance is up the steep ramp leading up from Rue des Clercs.