‘I will go before you and level the mountains. I will break the gates of bronze and smash the bolts of iron.
I will go before you. Isaiah 13:4-17.
I will straighten the crooked places. Isaiah 40:4; Isaiah 42:16; Acts 1:15; Luke 3:5.
I will break down gates of brass. Ps 107:16.
Source: The Treasury of Biblical Knowledge
I will go before you: This was a promise from God to Cyrus, similar to the one he made to his ancient people. Babylon had a hundred gates of bronze.
Source: New Illustrated Caribbean Bible Commentary
Ps 107:16.
Source: Hispano-American Interdenominational Translation
CROSS-REFERENCES
d 2242 Isaiah 13:4
e 2243 Isa 40:4
f 2244 Ps 107:16
Source: New World Translation
bronze gates…iron bars. The metaphor alludes to the conquest of a besieged city. The city gates were the most vulnerable point, so they went to great lengths to make them small and strong. Some ancient doors were covered in bronze, with elaborate designs, to protect the wood, strengthen the door, and make it as fire resistant as possible. The bars were the locks on the gates of the city. Even if they were made of iron, God would help Ciro to knock them down.
Source: The Bible of the Americas
I will level… That is, I will leave without strength; mountains… TM rugged places. Follow LXX/Q/Sir.
Source: Textual Bible IV Edition
Another possible reading is: the mountains
Source: The Bible of the Americas
Sir and Qumram: mountains.