The Romulan Senate chamber was a large, circular room located in the center of a concentric seaside city on Romulus. It was within this grandiose building that the Senate of the Romulan Star Empire convened.
Within the chamber, there was a large statue of the imperial symbol over the main entrance to the room. The symbol was that of a bird-of-prey holding the two planets of the Romulan system, Romulus and Remus.
The main body of the Senate sat in three rows near the entrance of the room. The Praetor and the Continuing Committee sat on the other side of the chamber. As of 2379, The floor in the center of the room had a map of the Empire that also showed the Neutral Zone; this area was used for anyone wishing to address the Senate. On the wall opposite the main entrance was a large display screen where the Senate could review information or communicate with anyone outside the Senate chamber.
In 2379, this chamber was the site of the assassination of the entire Romulan Senate with the exception of Senator Tal'aura, who planted a miniature thalaron radiation projector that killed her fellow senators.
The first Federation citizen to have ever set foot in the chamber was Captain Jean-Luc Picard in 2379, when he was invited there by Praetor Shinzon after he seized power upon orchestrating the assassination of the Senate. (Star Trek Nemesis)
A miniature of the Romulan Senate chamber was created by Rob Woodruff and helped Conceptual Artist Tom Southwell to storyboard the scenes which were set in that room. ("Storyboarding the Action", Star Trek Nemesis (Special Edition) DVD)
The sets for the interiors of the Romulan Senate chamber were constructed on Paramount Stage 32. (Star Trek Nemesis call sheets)
In a line from the Star Trek: Deep Space Nine episode "In the Pale Moonlight" that was in the "Final Draft" script but not the aired episode, Benjamin Sisko tells Vreenak that proof of a Dominion attack on the Empire would mean nothing if the Romulans waited "until there were Jem'Hadar soldiers standing in your Senate chamber before you decided that the Founders weren't your friends after all." 
In the script for TNG: "Unification I", the establishing matte painting (which was later re-used for DS9: "Inter Arma Enim Silent Leges") was described as an exterior view of the "Capitol city" of Romulus in daylight, which the Krocton Segment might have been implied to be a part of. Although they are both concentric, it is markedly different than the city around the Senate chamber shown in both "Nemesis" as well as in "Babel One"/"United"/"The Aenar" (both installments use the exact same CGI model). As "Nemesis" and "Star Trek: Enterprise" take place after and before "Unification", respectively, the different appearances of the "Capitol city" on the one hand and the city surrounding the Senate chamber on the other hand are difficult to explain with a simple change of the city-scape over time. It is, however, possible both shots (Nemesis/Enterprise vs. TNG/DS9) show different parts of the same city, which might contain more than one such concentric area, therefore retaining the "Capitol city" as the location of the Senate chamber.
In many of the Star Trek novels, including Vulcan's Heart, Titan: Taking Wing, Enterprise: The Good That Men Do, Enterprise: Kobayashi Maru, Section 31: Rogue, and the Vulcan's Soul trilogy, the Romulan capitol building is referred to as the "Hall of State".