The style developed within the historical context of wars and invasions in the Dark Ages; and, as a result is characterized by heavy walls with minimal openings and semicircular/rounded arches – particularly in the earlier period of Romanesque design.
The intrinsic characteristic of the style – the rounded arch is sometimes called a “Roman arch” which gives the style its name. Massive pillars were required to support the barrel-vaulted roofs. The style changed during the three centuries of use as building techniques evolved, in part, in response to the propensity of the heavy roofs of the churches to collapse – which eventually led to new construction techniques such as the rib vault and flying buttresses of the Gothic age. Doorways opened beneath rounded arches and often had a tympanum (semi-circular decorative wall surface) overhead. The buildings had large apses (part of a church shaped like half a circle, usually located at the far end with the main altar beneath) and, typically, had a large central tower, and sometimes, smaller adjacent towers – a free-standing tower campanile) was also sometimes used. Decorative patterns were usually geometric, repeating designs rather than curves as were used in the Baroque style.
Example illustrated: Santiago de Compostela Cathedral in Santiago, Spain
See image with heavy walls, small arched windows and large apse of the main church; along with an elaborate Baroque western façade.
Another example: Basilica of San Vitale in Ravenna, Italy