Here are some things to remember about keeping a horse in a field.

Fencing: A field should have a strong fence with no gaps or broken pieces.  Most people think that a post-and-plank fence is best.  Plastic fences are gaining popularity.  There are also welded wire fences.  They are made of squares so small that a horse couldn't catch its foot in them.  The fence should be at least 4 feet tall.  Electric fences will not keep a scared horse in.  Most people use them only as a scaring method.  You could use it as a divider for a large field so that you can rotate grazing ground once in a while.  If you use electric wire fence, tie colorful, plastic strips to it so the horse knows the fence is there.  Never put your horse in a field that has a barbed wire fence.  The horse could get very badly injured.

Gates: Gates should be as tall as the fence that surrounds it.  It should be made of a strong material, and have a horse-proof latch. 

Natural Hazards: Make sure that there aren't any holes in the ground.  Fill any holes that there might be.  If a hole is too large to fill, put a fence around it.  If you find any trees that grow fruit.  Make sure that when the fruit ripens to pick it before your horse gets to it.  Finally, look for poisonous plants and remove them.

Water: Your field should have plenty of fresh, clean water from a tap available at all times.  Otherwise your horse could get dehydrated.  It can be kept in a trough, plastic barrel, or an old bathtub.  You could fill it every day by hose, or have a plumber install an automatic waterer with a valve.  Check the water every day to make sure that the water hasn't frozen.  If it has, break the ice. 

Shelter: You should have some kind of shelter in the field.  Normally it is a barn or shed, but a clump of trees could do.  You need to have something to keep them out of rain and heat.

Shade: The field should have at least one acre of space.  If too many horses are in one area there wouldn't be enough space or grass.  If you have more than one field, rotate the horses so that you can rotate them for grazing.  That way the grass can grow back.