I'm gonna try and describe this as best I can:

On old sailing ships (i.e. Pirates of the Caribbean ships) there is a large box thing on the quarterdeck(?) near the helm, usually directly behind it. It's pretty big. My question is, what is this thing called, if it has a name, and what on earth is it for?

This is for use in the new epic. Hope you guys like nautical battles. 8D Anyway, thanks in advance to anyone that could help.