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The beauty of woodsball is that it can be played in almost any area, on almost any terrain and your field can adapt to wherever you want to play.
Choose a location that is relatively easy to get to, out of shot of houses, cars and busy roads, has natural cover (trees, rocks, hills. etc.) and is large enough to accommodate your group of players. A field should be large enough that you are not within range of the opposing team at the beginning of the game, but not so large that you are out of ear shot.
Before beginning, clearly establish boundaries. It may be enough to simply use natural landmarks, but you'll sometimes have to mark boundaries with plastic tape. Arrange the field so that two bases are on opposite ends of the field (these could be trees, rocks or any natural feature) where the opposing teams will start. Make sure to include a dead area where eliminated players can safely go while a game is still being played.
While not necessary, it is sometimes fun to add simple bunkers to your field. Bunkers can be made from a pile of rocks, branches stacked against a tree or from pieces of plywood that you bring. Don't be too concerned with bunkers as woodsball can be enjoyed with all natural cover. If you are playing on public property, be sure to dismantle bunkers after you are finished playing.
Other Things to Keep in Mind
Hills are very strategically important, so consider where bases are in relation to any rise in elevation.
Try and make both sides of the field comparable in terms of terrain. For example, don't put one team in dense brush and the other team in thin woods.
If possible, put the dead zone out of range of the field so people can safely remove their goggles after they are out.
Make sure you have permission to use the land you are playing on before you setup your field.