Partially attributed to the Flemish architect Jan van Ruysbroeck, it is a notable illustration of late medieval architecture in Brussels.

[4] This first collegiate church was in the Romanesque style, as attested by the 11th-century crypt that still exists beneath the chancel, and which was intended for the worship of relics.

[12] De Mol was in turn succeeded by Jan van Ruysbroeck, the court architect of Philip the Good,[12][13] who also designed the tower of Brussels' Town Hall.

The famous master builder Louis van Bodeghem [fr], the architect behind the Royal Monastery of Brou at Bourg-en-Bresse, France, designed the portal.

Under the Ancien Régime, during Pentecost celebrations, young peasants mounted on draught horses adorned with flowers and ribbons would mingle with the pilgrims who had come to implore Saint Guy.

[17] Restoration work was carried out between 1843 and 1847, under the direction of the architect Jules-Jacques Van Ysendyck [fr],[2][3] which lead to the discovery of several wall paintings from the early 15th and 16th centuries.

[17] In 1898, the square tower was surmounted by a neo-Gothic octagonal spire designed by Van Ysendyck, giving the church its current appearance.

[24] It is built of stone from Avesnes (northern France), as well as the Dilbeek quarry, which is located in present-day Flemish Brabant, approximately 4 km (2.5 mi) west of the church's site.

Built by Jules Van Ysendyck and inaugurated in 1898, it reflects the contemporary desire to glorify Belgium's national heritage, even if it meant adding to it when it was deemed incomplete.

[21] The nave, shaped like a Latin cross, has only four low bays, and extends beyond the transept into the relatively deep choir (including the canons' stalls) facing east.

[27][28] Its three vaults are divided by cylindrical columns topped with capitals in the shape of cabbage leaves, according to the Brabantine Gothic tradition.

[31][26] The two stained glass windows at the entrance to the choir, on the left and right, date from the 15th and 16th centuries, and represent respectively the Virgin and Child and the Intercession.

The nature of the stone, which is rough and slightly pinkish, lends credence to the legend that it was reused material from an ancient Roman villa, although petrographic analyses do not confirm this.

[36][37] In the centre of the crypt, a trapezoidal slab decorated with a lily of life, produced by Tournaisian sculpture workshops in the late 11th century, is believed to be the "tomb" of Saint Guy.

[7][37] The crypt's masonry was reinforced in the 15th century, when the current choir was built, and the 19th-century restoration gave the stones a homogeneous appearance.

[38] Around a hundred tombstones of canons and other illustrious people who died between the 15th and 18th centuries cover the floor of the transept, nave and aisles,[27][39] amongst which: On the north-west side of the transept is the funerary monument of Albert Ditmar, a renowned doctor, originally from Bremen (died 1438), who was attached to the court of Brabant and was called to the bedside of Everard t'Serclaes after his attack.