As Rome spread across Europe, from ~900-1200 AD, heavier, more massive architecture with rounded arches emerged. Churches and castles of this early Medieval period were constructed with thick walls and heavy piers. Even as the Roman Empire faded, Roman ideas reached far across Europe. Romanesque buildings had Byzantine-style domes and Gothic-like steeples. The typical floor plan is that of the Latin cross with a high altar and tower at the cross intersection. Large Romanesque buildings were made of stone and brick.
Example: Pisa Cathedral, Italy (c. 1092 AD)
First 10 image search results of romanesque architecture