The slope allows rain to run off and affords a better angle for gathering the sun's heat. Your cold frame should face south for maximum exposure to sunlight. Also select a location with a slight ground slope to provide adequate drainage away from the frame. To provide ventilation, partially open your cold frame during sunny, warm weather. During cold snaps, cover the cold frame with burlap or heavy cloth to provide extra warmth.
Hot Beds are cold frames with a source of bottom heat.
Today, that heat comes from electric heating cables. A few hot beds are still constructed using the old method of a layer about a foot and a half thick of decomposing manure beneath the soil of the cold frame as the source of heat. Either way the hot bed remains frost-free during the winter. There are perhaps as many designs for greenhouses as there have been imaginative gardeners. Unheated greenhouses are great for raising seedling crops of many of the most delicious leafy and root vegetables.