What abuts the Largo Argentina excavation is the eastern end of the theater complex, a part of the great back-stage public portico (all the comforts, including a multi-seat loo). Only a little bit is visible, behind the round republican temple (Temple B, see below), but that little bit is what remains of one of the most evocative buildings in Rome. This undistinguished looking wall is the back of a large exedra that Pompey added to his portico to house a new curia for Senate meetings. In 52 BC, almost as soon as Pompey's curia was finished, rioters protesting the murder of Clodius Pulcher by a rival gang lord burned the traditional Senate curia in the Forum. The Senate, under Pompey's control, made the move, and that is where it was meeting on that fateful Ides of March in 44 BC when Julius Caesar arrived with no -- or at least too few -- bodyguards. The front porch of the curia, where the fatal blows were struck, is not visible in the excavations: that's under the sidewalk in front of the Teatro Argentina, an historic site in its own right, where some of Verdi's operas were premiered in the 19th century.
The Campus Martius area was studded with temples built by returning victorious generals, whose armies camped on the flat river plain while waiting for the Senate to give them permission to enter the walled city in triumph. The Largo Argentina excavation, begun in 1929 as part of one of Mussolini's grandiose street widening adventures, exposed four such temples, which probably were ranged along one side of a street. It appears that Pompey enclosed the four in a portico attached to the eastern end of his own theater portico. The ruins of three of the temples are completely uncovered along with half of the fourth. Via Torre di Argentina runs along the western edge, between the excavation and Teatro Argentina, and traffic is also heavy on the shorter northern and southern ends, but Via S. Nicolo de' Cesarini to the east of the dig, facing the front of the temples, has a safe viewing area.
Although theories abound, there is no hard evidence to show to which deities the four temples were dedicated. They are universally known among archeologists as Temples A, B, C, and D (from north to south) of the Area Sacra of Largo Argentina.
The best preserved of the four is the rectangular Temple A to the north, which owes its standing columns to the fact that it was incorporated as part of a medieval church, S. Nicolo dei Cesarini (or Calcari = "lime burners") built in 1132. Parts of two Apses belonging to the church remain at the rear. At least two earlier temple-building phases were detected beneath Temple A.
Temple B is obviously the latest built of the four: its foundation is set even with pavement that had already been built up around the others. It is circular, and parts of six of its original 18 free-standing columns are in place. A raised circular plinth nearly filled the coaxial cella and supported a very large cult statue (identity unknown), parts of which were found between Temples B and C. The head, right arm, and a foot are displayed in the Capitoline Museum. Because of its shape, conformation and approximate location, this temple is the one that has the best identification. It is thought to have been dedicated to Fortuna in her guise of "Good Fortune for Today."
Temple C is both the oldest and the youngest of the temples. Tufa blocks visible at the sides of its foundation predate the lowest level of Temple A, but the floor of the cella and surviving superstructure date from a complete rebuilding after 80 AD. The colossal matronly goddess head, found nearby and now set up in the far northwest corner of the site, dates from the same period and may have come from Temple C.
The biggest of the four temples was Temple D, which is still mostly hidden under Via Florida to the south of the excavation. It has at least two phases, both of which were based on cement cores, so even the first phase is not likely to be earlier than the 2nd century BC.
The remains of some later buildings, which are only vaguely understood, lie among the ruins of the temples complicating the site even further. The long marble-lined multi-seat latrine clearly visible behind Temples C and D belongs to Pompey's portico rather than the temple area -- the seats all face west looking out through the colonnade offering a clear view of the portico gardens.
P.S.: 1. The names of landmarks in the "Argentina" neighborhood in Rome's centro storrico, (Torre Argentina, Largo Argentina, Teatro Argentina, etc,) they have nothing to do with the Rome's silversmiths, with Cardinal Francesco Argentino, nor, most certainly, with the homeland of Juan and Evita. They have everything to do with Johannes Burckardt, an Alsatian Bishop who was a VIP in the Rome of 500 years ago where he was known as Burcardo Argentinensis. The Argentinensis part of his name came from Argentoratum, the old Latin name of his native town, Nieder-Haslach, near Strasbourg. Burckardt was Papal Master of Ceremonies for five successive Popes (Sixtus IV, Innocentius VIII, Alexander VI [the infamous Roderigo Borgia], Pius III and Julius II), and, as Master of Ceremonies for the Great Jubilee Year of 1500, he was certainly famous in his own time. Burcardo made powerful enemies, and almost all memory of him was abolished after he died. Nonetheless, at least part of his name is memorialized in his old neighborhood. His palazzo, on the ruins of part of Pompey's Portico, now houses a theater museum and library.
P.S.: 2. Slightly off the subject, but too interesting to pass up, is the story of another theater, which was built in 53 BC (i.e., at the time when Pompey's theater was under construction) by Caius Scribonius Curio for his father's funerary games. It was actually a double theater -- two wooden theaters set back to back to allow for two simultaneous dramatic performances. But the two theaters were set on pivots so that they could be rotated, audiences and all, and when face to face they made an elliptical amphi theater ("amphi" means "on both sides") for gladiatorial games. Curio's "amphitheater" is the first one documented and he (or an architect he employed) may have been the inventor the concept. Pliny the Elder documents it in Book 36, section 117, of his Natural History, but I've only found it in Latin on the Internet.
Only sparse information about the Largo Argentina Sacred Area is on the Internet:
Comune Roma site: http://www.comune.roma.it/cultura/italiano/monumenti/monumenti/antichi/area_sacra_largo_argentina/#inglese
Koskimies: http://www.siba.fi/~kkoskim/rooma/pages/LARGOARG.HTM and
Maecenas: http://wings.buffalo.edu/AandL/Maecenas/rome/largo_argent/thumbnails_contents.html and
Caen U. model of Pompey's Curia and Temple B: http://www.unicaen.fr/rome/italien/geographique/curiepompee.html
Pompey Theater Project: http://www.geocities.com/theatrumpompei/home.html
Burcardo Argentinensis: http://www.theatrelibrary.org/english.htm
Double Pivoting Theaters -- Pliny (in Latin) Book 36 (scroll down to section 117): http://www.ukans.edu/history/index/europe/ancient_rome/L/Roman/Texts/Pliny_the_Elder/36*.html