The Romanesque period was from approximately 800 A.D. to 1100 A.D. The term Romanesque was first given to this type of architecture in the 19th Century due to it's similarities between the barrel vault and the Roman arch. Church buildings, art, and sculpture, were all used for the purpose to spread the Christian Gospel.
During this time in Europe there was a very large interest in religion. Large numbers of people traveled on pilgrimages to visit sites of saints and martyrs. People believed that holy relics had the power to do miracles. The routes to the more famous holy places, such as Santiago, became very well traveled and required larger buildings to hold the large crowds. The basilica style church could not hold the large crowds which were coming. They began to build churches in the shape of the Latin cross. The pilgrim would enter the church through the nave. They would then come to the area known as the crossing, which was under a groin vault, where the vaults of the nave and the transepts would intersect. The relics of the church would be held and displayed in the area of the high alter. The pilgrims would be allowed to view the relics from the ambulatory which allowed for a good traffic pattern for these large crowds. The more famous the relics a church held, the larger the crowds it would attract.
The architects also wanted to get away from using wood for the ceilings. They began to use stone ceilings on the new type of churches. Barrel or groin vaults were used in the ceiling. The stone was supported in the middle by the arch construction but was very heavy. The weight of the ceilings would tend to buckle the walls outward. This pressure outward is known as outward thrust. To support the walls, large piles of stone would be stacked along the wall in intervals to buttress (or support) the walls from pushing outward.
Due to the weight of the stone ceiling, the wall of the church had to be very thick. Windows had to be small to keep the strength of the wall strong. Because of this, the churches interior was dim. This was not solved till the gothic church design was used.