From A Wiki of Ice and Fire
The sept at Whitewalls is the castle sept of Whitewalls, the seat of House Butterwell.
Layout
The castle sept had inner and outer doors, and altars to each of the Seven.[1]
History
During the wedding tourney at Whitewalls, Ser Duncan the Tall finds his squire Egg in the castle's sept, where he discovers that the boy, actually Prince Aegon Targaryen, had revealed his true identity to Lords Butterwell and Frey, claiming that Prince Maekar was already aware of the plot and on his way to Whitewalls with an army. When Lord Ambrose's son-in-law, Tommard Heddle, finds out that Prince Maekar's son was within the castle, he comes at once to the sept and attempts to take Egg hostage against Butterwell's wishes, but Duncan kills him.[1]