One of the primary Amtgard activities.
A Ditch Battle is a battlegame without Class, without armor, projectiles, or (in theory) Role-playing. Its primary purpose is combat, and the practice thereof. Any melee weapon or shields may be used. By tradition, Red Weapons do not destroy shields in a Ditch Battle.*
In a Ditch Battle, you split into two sides. It doesn't matter if you split them evenly, it will work itself out in time. You can do this with just a handful of people or you can do it with over a hundred.
The two sides fight, with dead people staying dead and moving out of the way. Once one side wins, the victorious side sends the first person on their team to die over to the losing side. And you repeat. In time the better side gets whittled down until they are much smaller and the tide turns.
Between rounds, if new people arrive, they are added to the losing side instead of having someone switch. Generally all new people enter the same side at the same time no matter how many new people are arriving.
If the losing side did not kill anyone, no one moves, they have to try harder next time.
It is not uncommon for a team to make its goal to get one guy they want first, so even if they lose they'll get the person they want.
Some variants include weapon restrictions (e.g., "Single Sword Ditch" is everyone uses only single swords), wounds (the losing side gets first wound if no one is killed), No-Leg Ditch battles, and Militia Battles, which add armor and projectiles.
*The use of red weapons in ditch is the source of occasional controversy. A strict reading of the rulebook on the subject seems to show they are permitted. Great / red weapons with shield breaking weapons are a weapon category, not a class ability, and the rulebook only says that projectiles and bows are not used in ditch. However, by tradition, the vast majority of lands ditch without great weapons breaking shields in the core kingdoms, they have never, ever been used in ditches.