Already in the days of Tar-Minastir, the eleventh King of Númenor, he had fortified the land of Mordor and had built there the Tower of Barad-dûr, and thereafter he strove ever for the dominion of Middle-earth, to become a king over all kings and as a god unto Men.
(...)
But Sauron caused to be built upon the hill in the midst of the city of the Númenóreans, Armenelos the Golden, a mighty temple; and it was in the form of a circle at the base, and there the walls were fifty feet in thickness, and the width of the base was five hundred feet across the centre, and the walls rose from the ground five hundred feet, and they were crowned with a mighty dome.
...
And they sailed now with power and armoury to Middle-earth, and they came no longer as bringers of gifts, nor even as rulers, but as fierce men of war. And they hunted the men of Middle-earth and took their goods and enslaved them, and many they slew cruelly upon their altars. For they built in their fortresses temples and great tombs in those days; and men feared them, and the memory of the kindly kings of the ancient days faded from the world and was darkened by many a tale of dread.
- Silmarillion: Akallabeth
In the east and south well nigh all Men were under his dominion, and they grew strong in those days and built many towns and walls of stone, and they were numerous and fierce in war and aimed with iron. To them Sauron was both king and god; and they feared him exceedingly, for he surrounded his abode with fire.
- Silmarillion; Of the Rings of Power...
(concerning the Second Age)