The area just inside the doors is called the narthex. In the Middle Ages it was sometimes called the galilee because processions were organized here before they entered the main church. (Jesus was in Galilee before he made his entry into Jerusalem on Palm Sunday.)

The main body of the cathedral is called the nave. The area between the pillars and the outer wall is called the aisle. The aisle usually had its own vaulting and had a lower roof. Sometimes there was a gallery above the aisle looking down into the nave.

In medieval cathedrals, the altar was placed at the crossing, not at the back of the church like today. A candle high on the wall, called the sancuary light, was kept burning at all times, to symbolize the presence of God.

A row of pillars and arches, called the nave arcade, supports the side walls of the church.

The solid wall above the arcade is called the triforium and hides the roof above the aisles. This was usually decorated. (The Bayeux Tapestry was made to fit in a space like this in the Bayeux Cathedral.)

The space above the triforium is called the clerestory. Most of the light inside the church comes from the windows located here.

The area behind the altar is called the choir. Sometimes there was a small chapel built into the back wall, but I didn't have enough bricks for that.