For what reasons do Spanish castles and Romanesque temples resemble fortifications?

From the 11th to the 13th century, the Romanesque style prevailed in Western European architecture. Fortresses, monasteries, castles and churches were built on elevated terrain and resembled powerful defensive fortifications.
Endless military confrontations between feudal lords and wars with foreign invaders that took place at this time made great changes in construction and architecture. The rugged fortress-like structures served as a refuge during the wars. In the castles, residents of neighboring villages and livestock were hiding from the enemy. Around castles, monasteries, fortresses, a deep moat was often pulled out and fortress walls were built.