Albert Barnes Commentary 2 Kings 17:9

"And the children of Israel did secretly things that were not right against Jehovah their God: and they built them high places in all their cities, from the tower of the watchmen to the fortified city;" — 2 Kings 17:9 (ASV)

Literally, the words are as follows: “And the children of Israel concealed (or dissembled) words that were not right concerning the LORD their God.” The true meaning of this is likely that the Israelites cloaked their idolatry with the pretense that it was worship of the LORD. They glossed it over and were deceptive toward God, instead of openly acknowledging their apostasy.

From the tower of the watchmen to the fenced city—this phrase was likely a proverbial expression for universality, meaning, strictly speaking, “alike in the most populous and in the most desolate regions.” “Towers of watchmen” were built to protect the flocks and herds that were pastured in remote and desert places (2 Chronicles 26:10; 2 Chronicles 27:4).