The Architect's tomb is the tomb of the architect of Pandora's Temple, Pathos Verdes III. It is located at the very top of the Cliffs of Madness. The doors into the tomb itself are guarded by a series of moving platforms, with spinning buzzsaws running between them. When Kratos enters the tomb, he finds the final resting place of both Pathos Verdes III and his wife. Kratos then takes the architect wife's head, and uses it to unlock a doorway that leads beyond the tomb to Zeus Mountain, and ultimately to Pandora's Box.
Though named The Architect's Tomb, it probably wasn't originally intended to be so, as it is the sight of a murder and a suicide. More likely it was Pathos Verde's planning room and where he happened to be when he died.