The answer to this question can be found in Chapter 13, when the dwarves and Bilbo discover that they are trapped in the mountain and have no way out except to go through Smaug's lair. This is of course something that they are very concerned about, however, as they carefully make there way to the lair, it is obvious that Smaug is not present. It is then that they enter a massive chamber that has a ceiling that is high above Bilbo and the dwarves. Thorin is able to tell Bilbo what this chamber is and where it is in relation to the rest of this former dwarven city:
"This is the great chamber of Thror," said Thorin; "the hall of feasting and council. Not far off now is the Front Gate."
It is therefore Thror's hall that leads directly to the Front Gate in this location, and the size of this hall denotes is importance and prestige as the major part of the city where the dwarves met and drank and ate together. From here, Bilbo and the dwarves are able to reach the Front Gate and the exit of this ruined fortress.