Considering Lewis' own love of myth, legend and literature, I always figured it was a round table, like in the halls of Camelot in Arthurian Lore, Mount Olympus in Green Mythology, or Asgard in the Norse sagas, or even more currently, The Council of Elrond the Lord of the Rings, The Jedi Council in Star Wars, the Captain's Ready Room in Star Trek, The Justice League and The Avengers. Even if a table is not present in the room, the circular style of seating allows all members of these councils to have equal footing with each other and be equally seen and heard in the discussion, there you're the strongest/most powerful/wisest like King Arthur, Zeus, Odin, Gandalf, Yoda, Spock, Jean-Luc-Picard, Benjamin Sisko, Kathryn Janeway, Superman or Captain America or if you're "the least" like Sir Robin the Not-Quite-So-Brave-as-Sir Launcelot, Dionysus, Hod, Pavel Chekov, Wesley Crusher, Quark, Neelix, Frodo Baggins, Yareal Poof, The Flash, or Hawkeye. Here, the "First Son of Adam and Daughter of Eve" to enter Narnia, Lord Diggory and Lady Polly are on the same footing as the youngest, Eustace and Jill. Is there still "rank" or authority, yes, but it doesn't mean that the least members of these groups are shoved off to a side twiddling their thumbs.
What if...this round table was in a room at a Pub, say The Eagle and Child, and a certain Oxford don overheard them talking while he waited for his friends Tollers, and Williams to show up for a pint and thought to himself, " that would make for an interesting story..."