When you reach the city, press battle button instead of the enter one. You will be able to fight the city. With full party on Vanguard not chosen specially to fight I mostly win.
While I was making the bucaneers missions, when I have to get to a city while attacked by mercenaries/enforcerers, sometimes they let me go without fighting. Is it some kind of a bug?
,,Note: If you encounter lag please try the following - Right click on the game screen and switch storage settings to unlimited."
From game description.
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