Today, it is widely accepted by scholars that the round Temple of Mars Ultor depicted on provincial Roman coins in c. 19–16 BC refers to a schematic design of an intended structure, rather than the very different temple that was realised almost 20 years later in the Forum Augustum. It is argued that the planned rotunda was inspired by the Philippeion built in Olympia by Philip of Macedon as a dynastic monument, which was also an important purpose of the forum-temple complex built by Augustus. Possible reasons are offered for the change of design from a Greek-style ‚tholos‘ to a rectangular temple on a podium in Italic fashion. Is it possible that the original scheme for a ‚tholos‘ has left an imprint in the existing Forum Augustum?