Little is known about Temple of Janus (Aedes Janus) as it no longer exists, but it appears on Neronian coins. The temple stood along the Argileto, the ancient road that separated the Basilica Aemilia and the Curia Julia, where this road entered the Forum Romanum.
The Temple of Janus had doors to both sides, and inside the temple was a statue of the two-faced Janus. The doors were to be closed in times of peace and open in times of war. Being a warlike nation the doors were rarely closed, but it did happen during the reign of Augustus.
There is nothing left of the temple.
- LacusCurtius: Platner: A Topographical Dictionary of Ancient Rome
LacusCurtius: Hülsen: Il Foro Romano – Storia e Monumenti
(with a drawing of a coin with the temple)
Roman Numismatic Gallery: Buildings on Roman Coins
(includes one with the Temple of Janus)