This article focuses on Notre-Dame de Donnemarie-en-Montois and the manner in which its twelfth-century tower was integrated into the early-thirteenth-century church. Notre-Dame's position as a moderately sized collegiate church places it in a field of medieval art-historical research that remains relatively unexplored. Three issues are investigated: the building practices that allowed for such a fusion; the rationale for saving the tower; and how the concept of unity-often a driving force in Gothic architecture-was affected by this integration. This study was conducted with the help of a surveyed plan of the site drawn up by the author and an in-depth examination of the fabric of the church to separate original construction from subsequent additions and alterations. Knowledge of how the church developed over time is provided at the outset and acts as a foundation for reconstructing the manner in which each mason dealt with the tower. Although this feature appears to be well integrated, it occupies an awkward position, hovering over a bay and a half on the south side, and was never used as a module for the church. Yet retaining the tower may have helped in the choice of a hollow-wall system for the choir bays; it certainly provided the first master mason with a natural buttress system for his construction. Thus, the decision to save the tower might have been based on structural considerations as well as cost effectiveness, aesthetics, function, and political gain.