How should I design a system for an action game (think in Golden Axe) where events can happen remotely?

I'm using TCP for this because the client is in flash. There's so many options, I can make a binary protocol (I don't like this idea, I found it to be too hard to mantain) but I was also thinking that passing jsons through clients and server can be slow (Is that a exaggerated concern?).

What about the internal architecture for the server? And for the client? I'm really lost, If it's a question that is too big, please indicate me some material so I can formulate a better question next time.