Yeah, that's the highest level by default. There are mods that were made that made the highest level stage 16, though.
And the "online" button in the game is really just minimizing the game and opening a web browser and going to the official game site for custom content (a site which has been dead pretty much since 2004). The fansites, however, filled the void quite readily. I prefer to play with mods, myself, because they're usually game experience enhancers.