As is typical in the region, Sainte-Catherine in Monceau-sur-Oise is largely built of brick. The foundations are made of sandstone, as are smaller parts of the side and choir walls. However, these are more indicative of repair work.

The two high towers, which frame the entrance portal and are perforated with embrasures, date from the 16th century, when the church was fortified. There was a third tower at the rear of the building, but it was destroyed in the First World War.

The choir of the church was restored in 1671 as part of the second major renovation campaign of the Thiérache churches. Finally, on the façade, the large rose that surmounts the portal was probably also altered.

Unfortunately, the church was closed when we visited in autumn 2023, although it is normally open at weekends in summer. However, renovation work was probably also taking place.

Incidentally, the village has another special feature in addition to the church, which is also pointed out on the village sign and has nothing to do with fortified churches: it was one of the first municipalities in the Aisne department and perhaps also in France to have public lighting, as an electricity plant was put into operation in the village in 1905.