The question of original floor levels in medieval church buildings came up in discussion on another post. Generally speaking the great ranks of steps you see in many of our medieval churches were introduced during the Gothic Revival. In the Middle Ages only a modest rise in floor level was usual from east to west, the high altar being perhaps two, at the most three steps above the level of the nave. Side altars were often only placed on a single step above the level of the aisles they were set in.
There are of course exceptions, one is the altar placed at the east end of the south nave aisle at Dorchester Abbey in Oxfordshire. This chapel was the parish church of Dorchester in the Middle Ages and was known as the ‘people’s chapel’. The original liturgical arrangements of this chapel survive, including an early fourteenth century painted reredos, piscina and sedilia. The altar was raised above the level of the aisle on four steps. In this case the steps served a practical purpose, as they covered a vaulted charnel house containing the bodies of those disturbed during the construction of the aisle.