Final answer:
They built castles, walls, and moats for protection from enemy invasions, often combining these elements to create a well-fortified defense system.
Step-by-step explanation:
Throughout history, various methods have been employed for protection from enemy invasion. Castles were fortified structures designed to serve as a residence for nobility and as a military stronghold. Surrounding a castle, walls were erected to prevent enemy forces from easily penetrating the heart of the domain. Additionally, moats, which are deep, wide ditches filled with water, often encircled castles to further dissuade and hinder attackers. The comprehensive approach to defense typically included all of these elements, creating a formidable barrier against invaders.
Therefore, the correct answer to the question, "What did they build for protection from enemy invasion?" is D) All of the above, as they represent the combined efforts of past societies to safeguard themselves from external threats.