This giant tomb was built between 18 and 12 B.C.E. for Caius Cestius who was both a magistrate and member of the Roman religious corporation known as Septemviri Epulonum, which was in charge of creating religious feasts for the gods, and distributing swag during parties. During the construction of the Aurelian Walls between 271 and 275, the pyramid was built into the bulwark, forming a triangular bastion in an effort to cut costs on the snowballing wall project.
You'll notice the faces of the pyramid are far steeper than its Egyptian ancestors. This is attributed to erroneous sketches of the Egyptian pyramids which acted as the chief reference for its construction.