Ok now, look at that *very large structure* on your left, that you are standing in front of, shown in this photo. In guidebooks it's usually called 'Domitian's Hall' or 'Hall in Opus Latericium' (26 m buttressed walls in brick-faced concrete), it is also signposted *wrongly* as the 'Temple of Augustus' which it was believed to be in a nineteenth Century excavation.
Let's start from before this Hall was built. On top of the Palatine Hill in front of you was Tiberius' Palace complex (By the way, the word palace comes from Palatine) and after Tiberius' death (14 - 37 AD) Caligula (37 - 41 AD) moved in. And he built the first extension outward from the Palace area called the 'Domus Gai'. On top of this extension he had a garden, think of it like a patio to his palace complex. After Caligula was murdered the guards entered the palace complex and killed his wife and infant daughter. His uncle Claudius (Emperor 41 - 54 AD) in fear of his life hid in *a* garden first but then went into the palace area and hid behind a curtain (like a comedy act his feet stuck out and he was discovered :-) . I wonder *if* that was the garden?
Emperor Domitian (81 - 96 AD) built over the Domus Gai and it's his larger extension that we see today. (If you have an interest in these huge extension-platforms visit the opposite corner of the Palatine Hill and be awed by the 'Severan Arcades' which must be seen at their base) But he was assassinated before it was completed if it ever was? It might have been roofed over (wooden). Also the front facade (arcades or columns) has either been lost or was never built. The interior was meant to be lined with marble framing the tall niches in the walls but it never was finished. But then again it might have had a vaulted roof and the walls were lined in marble. As you can see the sources differ and differ again :-) .
This Hall has been wrongly identified (wrong location) as Hadrian's Athenaeum (117-138 AD). Or *it* was called an Athenaeum (library) when Domitian built it because this was the site of a Cult of Minerva and she was the patron goddess of scholarship and this Hall was the 'Sanctuary of Minerva'. Here military diplomas with the names of soldiers who retired with honors were displayed. Or it might have been a vestibule entrance to the palace above but never finished.
And later under Hadrian this unfinished shell of a building was converted into a Horreum (small warehouse) with storerooms and offices along the walls and a paved courtyard in the middle, making it in essence part of the Horrea Agrippiana. The five short walls in front of the Hall are the remains of shops which lined the Vicus Tuscus. And behind the far wall was another smaller room with a portico. In the sixth Century the Church of Santa Maria Antiqua was built there and still remains. Inside are rare seventh to eighth Century Christian wall paintings.
As you leave this area be sure to look between the Temple of Castor and Pollux and this Hall, you will see the 'Oratory of the Forty Martyrs' in the back that we will get to later.
Also imagine Caligula blocking this street and building a staircase down to the Temple of Castor and Pollux as a shortcut from the Palatine so he could converse with the Gods Castor and Pollux.
For more information and photos, please see Domitian's Hall in A Tourist in Rome.
Next: #19: Temple of Julius Caesar