Past it being a field, this location did not exist before the apocalypse.
After Woodbury had grown a sizable population, Philip Blake decided that his people needed a way to safely see the horrors of the outside world. He decided to have gladiator fights with walkers, however, in order to achieve this he needed to capture said walkers. He made this location in a nearby field so that he can get the necessary walkers. As a added benefit to this, Milton Mamet, his head researcher, would also be able to study the walkers up close.
Milton, Merle, Martinez, and Tim drive a tow-truck to an off-site pit, where they hoist out a netted trap full of Walkers. Merle taunts the Walkers, then holds one down before pulling its teeth out with pliers.
In the middle of the night, an unknown person douses the pit walkers in gasoline and lights them on fire.
The next morning, Shumpert arrives at the pit and finds the scorched walkers.