The fire chamber had two gates: one opened to the cheil (the area around the courtyard) and the other to the courtyard. Rabbi Yehuda said that the gate that opened into the courtyard had a smaller gate through which the kohanim would enter to inspect the courtyard (as described in Tamid 1:3).
The fire chamber was domed. It was a large room ringed by stone slabs on which the elders of the duty rotation would sleep, guarding the keys to the courtyard. The younger kohanim would sleep on their cushions on the ground.