Yeah but by 1800 ships could massively damage cities using rockets and cannons, and by 1900 the British could level a city. Air-cannons might be less penetrative then gunpowder but they also can fire explosive or incendiary rounds with less problems, so a massive armored ship just sailing into harbor and destroying your docks would be a real threat.
On that note; in RL Ironclads were cannon proof until a decade into smokeless cannons. Cannons that could penetrate their armor had a tendency to explode (the US Secretary of War was killed in a testfire for instance), and they were built to ram each other until the late 1890s because of this.