The Arsenal is a citadel protecting the main harbor of Braavos. It is located at the entrance to the lagoon, just beyond the Titan of Braavos. It sits upon a knob of rock, fortified with stone battlements and bristling with scorpions, trebuchets, and spitfires. 
The Arsenal is also a dock with room for dozens of galleys beneath it. Along it's shores are innumerable quays, docks, and wooden sheds holding many more ships. It houses the defense fleet of the city and is the center of shipbuilding in Braavos. It is said that a war galley can be built in a day there.