The megaron was the great hall of the Mycenaean world, with a universal, three-part floor plan.
Visitors would enter through a columned porch called an aithousa; this was also where Telemachus and Peisistratus in The Odyssey slept when visiting Menelaus's palace in Sparta. Mycenaean palaces probably did have a guest room or two for important people, but remember that the two princes arrived in the middle of a royal double wedding, when all the available space was probably taken. The mistress of the house would make certain that guests sleeping on the aithousa were furnished with blankets and warm fleeces, and that they were fed and washed according to the strict codes of hospitality.
No one entered the megaron after-hours.
Double doors led from the aithousa into the vestibule or prodomos, an area where important petitioners or ambassadors might wait on benches. A curtain separated the vestibule from the domos, or main area--the megaron itself.
Inside the megaron, a fire burned on the central hearth with its four pillars. The throne sat not opposite the doors, as you might expect, but on the right as you entered. Often, there was a second-floor gallery where the court ladies might watch the proceedings.
As to what the megaron looked like, let's start with this famous Piet de Jong reconstruction of the Pylos megaron (throne room), found by Carl Blegen in the 1930s. Only the first floor survives, with its hearth (still bearing traces of its painted decorations 3,200 years later), the pillar bases, the floor decorations done in painted stucco, and the dais where King Nestor's wooden throne would have sat.
De Jong's illustration is deceiving, because anyone who has ever visited the archaeological site of the Palace of Nestor in Messenia knows the real megaron is much smaller, only 11 meters wide. It also would have been darker, as there were no windows, and the only light would have come from the hearth. It would have been quite smoky as well, especially in winter, as seen in this more realistic painting by Alton Tobey. Imagine yourself looking over the king's shoulder; the man at his right is sitting in the place of honor.