It's not an engine, but I found the SFML library to be fairly user friendly when working with 2D. It's muti-platform, has bindings for c# (I've only used with c++ so can't speak to it's c# capabilities at all). It has windows, audio, networking, and graphics modules. It uses a zlib/png license, which is free for commercial use. It has pretty much everything you'd need for a 2D game.
I found these tutorials helpful when starting out with it:
They're not perfect, but they're pretty comprehensive.
Many people opt for the SDL library instead, or one of many others, so maybe check out your choices.
For a 2D rpg, I would think that an engine would be largely unnecessary (as there isn't much in the way of physics and such. You're just moving a sprite around the screen, interacting with objects.) If you're going turn-based, it's even more unnecessary, I would think.
And, along with the comments above, an RPG, while from a programming standpoint isn't terribly complicated (depending on complexity of course), the sheer volume of the artwork, dialog, and other assets can be pretty daunting. It's just so heavily dependent on producing lots and lots of content. But, having no knowledge of your experience, feel free to disregard this comment Also, if you've already decided on a library (or doing without one) and specifically were looking for an engine, feel free to disregard all this too
If you really just want an engine, I sadly don't actually know of any decent free ones. There are things like rpg-maker and game-maker and such, but I don't believe they fit all of your requirements (c#, free, etc). But, perhaps someone else can be of more help in that regard.
Edited by Misantes, 18 May 2014 - 01:47 PM.