Interesting that you said you castle after White plays 5.0-0. That is a mistake. After d4, exd4 cxd4 you need to be able to play Bb4+ to gain and tempo and stop White's centre from steamrolling your Knights out of position. If White has castled that is no longer possible and exd4 becomes another mistake, leaving White much better.
Instaead of 5.0-0 0-0 you should play Nxe4 and Black is better.
Of course White will normally play 5.d4 or d3, not 0-0.
If White plays d4, then you're in fine shape after exd4.
In my opinion 5.d3 is White's best. However, nothing really happens for ages and I'm guessing that whoever named this opening had that in mind. I can't show you any forcing variations, you just move pieces around for a while. White tries to prepare a future d4; Black prepares for that future. Both players can bring their c Knight over to the Kingside. Both idealy want to land a Knight on f4/5 etc. I'm no expert.
I see it a good amount of times in my rating category. I want to learn more about it as black. Particularly when 4. c3. I usually play nf6, and after both sides castle, d4 and I don't really know what to do next. I usually take with e pawn.