Albert Barnes Commentary 2 Kings 15:35

"Howbeit the high places were not taken away: the people still sacrificed and burned incense in the high places. He built the upper gate of the house of Jehovah." — 2 Kings 15:35 (ASV)

He built the higher gate - Jotham followed his father's example in military as well as religious matters (Compare 2 Chronicles 26:9). The “higher” or “upper gate” of the temple is thought to have been the one on the north side, and its fortification seems to indicate a fear of attack from that direction.