This basilica was designed by Juan de Herrera after drawings by Italian architects, which is probably why the interior owes so much to St Peter's in Rome: a Greek cross plan, a 92m-high cupola supported by four colossal pillars, and barrel vaulting in the transept. The flat vault in the theatrium is an architectural novelty. The Spanish architect also signed the massive 30m-high altarpiece and the tabernacle. The mausoleums of Carlos V and Felipe II lie on either side of the chancel.