The Entrance Hall is where people enter Mainframe's Principal Office. It is a massive room with a very high ceiling. Once a person enters through the large gold doors of the Principal Office, they walk down the Entrance Hall to another set of gold doors that lead to the rest of the corridors in the Principal Office.
Phong has a podium in the Entrance Hall where he can great visitors and help them with their problems. His podium has a number of controls that Phong can use to access many of the controls for the Principal Office. Enzo brought a group of Mainframers there to ask Phong what they should do about Dot and Bob fighting. (The TIFF)
During the infection of the Medusa Bug, Mainframe's population was evacuated to the Principal Office and set up a small camp inside the massive Entrance Hall. The buildings defenses failed and the Principal Office turned to stone. The citizens watched as the Hall turned to stone and the virus came for them. (The Medusa Bug)
Dot and the second Bob wanted to get married in the War Room, but Cecil convinced them to get married in the Entrance Hall. Dot and Bob said that it was a little spartan and big, but Cecil said he would make it beautiful for a wedding. He filled the Hall with several church pues, lots of draperies, a pipe organ, statues, a pavilion at the alter, with an Icon symbol over it.
The beautiful decorations that Cecil put into the Entrance Hall were destroyed when the second Bob was revealed to be Megabyte in disguise. Bob and Megabyte fought in the middle of the Hall, destroying most of the beautiful decorations. (Null Bot of the Bride)