Unlike Sacré-Cœur, Saint-Denis is quite old, the current structure dating to the 12th century—and I understand there was an abbey on this site for a few centuries before that. It is interesting for two things: (1) it is considered the first structure to include all elements of Gothic architecture, and (2) it is the resting place of nearly all the kings of France, from the 10th century all the way to Louis XVIII.
Other fun facts: French queens were crowned here, while kings were crowned in Reims. Also, it was here that Henri IV formally renounced his Protestant faith to become Catholic so he could take the throne. As he pragmatically said, “Paris vaut bien une messe” (“Paris is well worth a Mass”).