Surprise! Football is period, although our ancestors would probably have sneered at all the rules and regulations we've added to our game. Medieval football was called "Gameball", `game' being the Old English word for fight or battle. The rules were much simpler ... there weren't any! First team to get the ball into their opposing team's goal, wins.
The ancient Greeks and Romans played "Harpastum", a game which combines elements of Soccer, Rugby and American Football. The Romans imported this game into England during the time of Julius Caesar; however it wasn't until the Middle Ages that the game really caught on.
The sides were seldom equal. There were no limits to the number of players. Women as well as men played. In a friendly pickup game among knights on a campaign, the goals were probably only a few dozen yards apart. In regular games between neighboring villages, with everything the phrase `traditional rivals' implies, it was not unusual for the goals to be hundreds of yards or even one or two miles apart.
The ball was a pig's bladder, at first stuffed with dried peas and later inflated with air. We still refer to a football as a "pigskin" even today, when modern footballs are made from cowhide. From Sports and Pastimes by Strutt, originally published in the early 1700's. "The ball, which is commonly made of a blown bladder and cased in leather, is delivered in the midst of the ground, and the object of each party is to drive it through the goal of their antagonists, which being achieved, the game is won." The illustration is a woodcut from Strutt's book showing a football being inflated.
A neutral person would throw the ball into the air as high as possible between the two teams and then run for dear life as the teams converged. There was no penalty for roughing the ref. The ball could be carried, kicked, passed or thrown, whatever got it closer to the goal.
The medieval version of the Superbowl took place on Shrove Tuesday, the day before Ash Wednesday. Entire towns participated and some games literally lasted for several days. The tradition continued off and on from the twelfth through the nineteenth centuries.
As you may have realized, the potential for for mayhem was enormous. Documented instances of crippling injuries, including several fatalities, abound. A papal dispensation was issued in 1321 to a player who accidentally killed an opponent. Kings Edward II, Edward III, Richard II, Henry V, Henry VI, Lord Oliver Cromwell, and Queen Elizabeth I, all prohibited the game due to the number of injuries it caused and because it took time away from archery practice. The laws were ignored.
In the mid-fifteenth century, a variation called "Kicking Camp" developed. The ball could ONLY be kicked, not carried or thrown. This game later developed into European football, called Soccer. The game of American Football evolved from the original game of "Gameball" which continued to be played right up to the Renaissance.
Football History -- http://wwwwbs.cs.tu-berlin.de/user/tiny/fhistory.html
The History of English Football -- http://www.the-english-football-archive.com/football_history.htm
The History of Football -- http://www.nenyl.org.uk/history_of_football.html
Reeves, Compton Pleasures and Pastimes of Medieval England (England; Alan Sutton Pub.;1995; ISBN 0-7509-0089-X; 228pgs) Barnes and Noble recently bought the printing rights for this previously out-of-print book.