Transgression is the same as sins. These two words describe Israel's failure in the Old Testament.
What is … what it: The capital cities of God's people should have been holy places, but they became sources of corruption instead. Samaria, the capital of the northern kingdom of Israel, was built by Omri, who ruled from 885 to 874 BC. It was a political, military, and economic crossroads of the ancient Near East (1 Kings 16:24). Omri was an evil king, so his city was evil (compare Micah 6:16, 1 Kings 16:25).
Jerusalem: The prophet did not allow the people of Judah to feel superior about the northern kingdom’s coming destruction. Judah’s beautiful temple was no different from a Canaanite center of idol worship (high place).