The undisputed centerpiece of this très jolie little town is the disproportionately large and beautiful Our Lady of the Assumption Co-Cathedral, built in 1919 in a Romanesque and Italianate style. It was designated a National Historic Site in 1995. Enter if it's open and crane your neck to marvel at the Sistine Chapel–esque frescoes. Monsignor Maillard, who not only designed the chapel's interior and presided over the parish, painted the frescoes himself, from 1921 to 1931 – an astonishing feat.
Tours can often be arranged through the Gravelbourg & District Museum.