The crypt of Santa Leocadia is one of the oldest surviving remains of the group of palatial and ecclesiastical buildings that existed in the area in the time of Alfonso II; the chaste of which the church of San Tirso also forms part, although the construction of the crypt is attributed to Alfonso III.

The crypt formed part of a set of two superimposed sanctuaries, but without communication between them; the crypt is the lower sanctuary and the upper sanctuary is the Holy Chamber.