The Top Nine is a king-of-the-hill tournament: each iteration contains only the best nine robots from the previous tournament, plus one challenger. This concentrates the good sides, and shows how a side performs against good opponents, rather than how well they can eat helpless targets.
The tournaments are run in a standard 10x10 world with ten sides for at least 200 (previously 100) rounds. There are considerable variations in the results, which could mean 100 rounds is not enough. This could also be due to effects sides have on each other - with only ten sides, replacing one can substantially affect the others. So don't take the results too seriously.
The pre-wiki history of the top nine tournament is available at the old Grobots webpage.
Current top nine: (TODO)