Churches were frequently built over ancient sacred Pagan sites. The Christian altar was placed on the East side of the church, or sometimes the West side of the church so the congregation would face East. However, a Pagan altar was sometimes included and was placed by the north door.
The north door was also referred to as the "Devil's Door" and was implied to be where the devil was let out to keep the congregation pure. This shaky reasoning may be a simple smokescreen. At the time these doors were in use, all Pagans were thought to be devils, or agents thereof. The most likely purpose of these doors was to integrate local Pagans to the church which was already built on top of their sacred site, with hopes to convert them over time.
Churches in as late as the 11th century had a Pagan altar. These north doors of most Norman churches were walled over from the 1300's onward as Witches were less tolerated.
Many magickal circles are cast with a 'door' placed in the North-East for entering or exiting. Some are created with an altar facing North or East. Look into the attributes of the cardinal points and elements to determine the placement of yours. Above all else however, do what really feels right to you no matter what direction you face your altar.
We like to vary the direction our altar faces depending on the work being done or the Sabbat being celebrated. A nice technique for larger circles is a round altar in the center of the circle. In this way each covener can utilize the altar equally, sort of the 'round table' effect.
For further reading on this and related topics, see "The Practice of Witchcraft Today" by Robin Skelton, published by Robert Hale Limited