BG3 Iron Throne prison walkthrough: ... To get to the Iron Throne in the first place, make your way to the Flymm’s Cargo warehouse on the eastern side of the harbour in Baldur’s Gate.

Redhammer is not a Banite or enemy of the Iron Throne's prisoners; rather, his personal notes express that he does this job for gold and out of fear of what would happen should he turn Gortash down.

You Should Briefly Visit The Steel Watch Foundry First In BG3 The Source Of Gortash's Security In Act 3, Karlach wants revenge against Gortash, who is the reason she ended up in Avernus.