During Bible History, high places were usually natural (e.g. hills and mountains) but sometimes man-made (e.g. towers) places where people worshiped or made sacrifices. Although the only True God often appeared, or was sometimes worshiped, atop mountains (e.g Mount Sinai) or hills (e.g. Samuel i.e. 1 Samuel 9:19), the Canaanite "high places" were notorious for pagan worship where various heathenistic idols, altars or buildings were located, and where some of the most wicked and horrendous behavior imaginable went on. As such, when the Israelites entered the Promised Land they were commanded by God not only to have nothing to do with high places, but to destroy them because they would be corrupted by their presence - a command that was largely disobeyed, to their regret.
As they were about to enter the land promised to the descendants of Abraham, Isaac and Jacob, God instructed the Israelites to demolish the idolatry in the land:
"And The Lord [see Rock Of Ages] said to Moses in the plains of Moab by the Jordan at Jericho, "Say to the people of Israel, When you pass over the Jordan into the land of Canaan, then you shall drive out all the inhabitants of the land from before you, and destroy all their figured stones, and destroy all their molten images, and demolish all their high places; and you shall take possession of the land and settle in it, for I have given the land to you to possess it." (Numbers 33:50-53 RSV)
The Command Disobeyed, The Israelites Corrupted
Often led by their own kings, the Israelites tolerated the pagan high places, and were themselves drawn into its evil:
"Manasseh [see Kings of Israel and Judah] was twelve years old when he began to reign, and he reigned fifty-five years in Jerusalem. His mother's name was Hephzibah. And he did what was evil in the sight of The Lord, according to the abominable practices of the nations whom The Lord drove out before the people of Israel. For he rebuilt the high places which Hezekiah his father had destroyed; and he erected altars for Baal, and made an Asherah, as Ahab [see Jezebel] king of Israel had done, and worshiped all the host of heaven, and served them." (2 Kings 21:1-3 RSV)
"Now King Solomon loved many foreign women: the daughter of Pharaoh, and Moabite, Ammonite, Edomite, Sidonian, and Hittite women, from the nations concerning which The Lord had said to the people of Israel, "You shall not enter into marriage with them, neither shall they with you, for surely they will turn away your heart after their gods"; Solomon clung to these in love. He had seven hundred wives, princesses, and three hundred concubines; and his wives turned away his heart. For when Solomon was old his wives turned away his heart after other gods; and his heart was not wholly true to The Lord his God, as was the heart of David his father. For Solomon went after Ashtoreth the goddess of the Sidonians, and after Milcom the abomination of the Ammonites. So Solomon did what was evil in the sight of The Lord, and did not wholly follow The Lord, as David his father had done. Then Solomon built a high place for Chemosh the abomination of Moab, and for Molech the abomination of the Ammonites, on the mountain east of Jerusalem. And so he did for all his foreign wives, who burned incense and sacrificed to their gods." (1 Kings 11:1-8 RSV)
Fact Finder: Was Solomon's idolatry the reason that God caused the division of Israel into two kingdoms?
1 Kings 11:9-13
See also Rehoboam