The Architect's tomb was the tomb of the architect of Pandora's Temple, Pathos Verdes III. It was located at the very top of the Cliffs of Madness. The doors into the tomb itself were guarded by a series of moving platforms, with spinning Buzzsaws that ran between them. When Kratos entered the tomb, he found the final resting place of both Pathos Verdes III and his wife. Kratos then took the Architect's Head, and used it to unlock a doorway that lead 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 was 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.