Epic Poetry
Since it deals with the rise and fall of a kingdom over a period of many years, the scope of Idylls of the King is definitely epic. Tennyson’s approach to epic here is a little bit unusual, though, because he divides this epic into a bunch of short stories focusing on just one or two people, which emphasizes the point of view of individuals involved in their own personal dramas. That’s a very different focus than we normally see in epic poetry, which tends to place individuals in the context of the big picture rather than the little picture, as Tennyson does.
Tragedy
The Arthurian kingdom is doomed to fall from the very beginning by the human nature of the people who form it. What binds it together is the knights’ unity with Arthur, sealed with an oath. But despite Arthur’s belief that “man’s word is God in man” (“Balin and Balan,” 8)—that is, a man’s oath is an omnipotent power that binds him and rules all his action—most of the characters in Idylls simply aren’t able to remain true to oaths of any kind. This means that the fall of the fellowship of the Round Table is a foregone conclusion at its beginning, making the Idylls a tragedy, pure and simple.