Thought I would go ahead and post a little about the architecture I have set up for my PBBG engine so far. Basically, the engine is a library that other projects can reference to make building PBBGs a little easier. All of the logic for handling incoming requests and parsing commands is contained within the engine. My PBBG engine is kind of like a MUD driver, it requires a MUDLib to define objects, persist data, etc. Some interfaces are defined within the engine library to make it extensible while it also contains base class implementations of those interfaces to make building on top of working logic possible. When the engine starts up it raises a set of events and when it processes commands it also raises events to allow the implementing application to control loading of commands, determining the server implementation and creating command instances.
Not the best diagram in the world but kind of gives you the idea of how it works. The column down the middle are the interfaces defined by then engine, which also define base classes, except for the CommandManager which is just a static class for processing commands.
In addition to the classes here are the Verb abstract class, the ICommand interface and the Command<Verb> instance for creating commands from Verbs. The CommandManager raises an event when it finds the command text that allows an external library to set the ICommand instance that will execute the current command. When the engine starts up though it raises an event that will allow the implementation library to set all the commands into memory rather than creating a new instance with each command.
The IServer interface defines an IConnectionManager property where connections into the server are managed, either socket or http.
On the NET side, which is actually a namespace ".Net"; the "Server" instance actually contains the socket used to listen for incoming connections on a port specified during startup. From there the individual IConnection instances contained in the IConnectionManager instance handle processing the input and formatting it into a command. The IConnection instance contains the connected socket and will send the command to the CommandManager.
Both IConnection instances in both namespaces allow messages to be sent to the client along with custom data such as stats, gold, map data, etc. The difference between the two is that the WEB instance sends all of this back as JSON in the HTTP response and the NET instance sends the messages down the socket as they are added to the IConnection instance and then sends the additional data once the command completes execution.
That is all I have completed at this point on the engine side, it processes commands from the web and the implementation library sitting on top of it handles the commands and persists the data. There is plenty of work on the implementation library side still yet to be completed.