Head down into the Royal Crypt using the entrance in the Basilica. Get to the first East/West Corridor and head to the West. Get into the large room after that.
Once inside the room overlooking the temple entrance, turn to the right and climb up the scaffolding. Head to the North then follow the beams to go West across the Northern wall. From there, cross over to the first pillar nearby the wall. Scale around to the Southern side of it and use the wooden beams to cross over to the Southern pillar.