An aisled transeptless basilica terminated to the east in a hemicycle with five shallow radiating chapels and ambulatory; the axial bay is wider (like Saint-Martin-des-Champs). The chapel at the base of the hemicycle on the north is vaulted with the ambulatory like S-Denis. To the west of the hemicycle comes a straight bay with quadripartite vault flanked by towers. Then comes a double-bay with sexpartite vaulting and alternating supports followed by six nave bays with quadripartite vaults and piliers cantonnés. The three-segment western frontispiece is considerably narrower than the body of the church and is not quite aligned.
A three-story elevation with arcade, triforium (looking like the gallery of Notre-Dame of Paris or Mantes) and clerestory.
S-Leu is located on a dominant site overlooking the river Oise close the the important quarries which provided building stone for an area extending to Paris and beyond as far as Chartres. The Priory was a recent foundation: in 1081 Hugues, Count of Dammartin gave the church on the site to Guy, bishop of Beauvais on condition that the bishop give it up to the monastery of Cluny for the establishment of a priory. There are no documents for the construction of the existing church.
There are three chronological entities in the church. First, the western frontispiece with its upstairs chapel in the Carolingian tradition. Around 1140 it was decided to extend the older (11th century) church to the west with a new frontispiece. Bits of the 11th century church are incorporated in this work. Second, around 1160 the builders abandoned their original plan in favor of a wider edifice, moving construction down to the other (eastern) end of the edifice with the construction of the chevet including the sexpartite-vaulted bay. Third, the nave was constructed between c.1180 and 1210. It was decided to conserve the old ill-fitting frontispiece.
The chevet of S-Leu with its shallow chapels refers both to S-Denis and to Senlis Cathedral--the plan and dimensions are particularly close to the latter. Unusual at S-Leu is the high chapel (added) over the axial bay of the hemicyle and the middle level of the elevation that looks so much like a gallery but is in fact a triforium.
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