By “the vestibule” is meant “the outer hall of the Baptistery” (xix. 2), and by “the King’s Palace” the Baptistery itself, which Cyril calls “the inner chamber” (xx. 1) and “the bride-chamber” (iii. 2; xxii. 2).  See Index, “Baptistery.”  Here the local terms have also an allegorical sense, Baptism being regarded as the marriage of the Soul to Christ.