Shelter
Man-made constructions need to be three-sided to allow for protection from the elements, and also to allow the horses to get in and out easily.
The field must have some kind of shelter. Trees and hedges form the best kind of natural shelter.
They need to be situated on ground that drains of well, and to be out of the prevailing winds. Make sure that the shelter is more than big enough.
Field shelters must be inspected, and the roof needs to be looked at for soundness and waterproofing.