Why is 10. O-O considered a brilliant move

Hi, just had one of the best games I've ever played in a 30 minute, with the Traxler counter-attack. I know it is a really sharp line, and if any mistakes are made the tables immediately turn, but I've had a lot of fun studying it recently. I was just wondering why is 10. O-O considered a brilliant move and not a best move like the others. I played it because I saw it in a few of the games I studied for the traxler in the king march line. Is there another move here that could maintain the same equality?

Also is it worth it for me to study the Traxler counterattack at this level?

I play 4...d5 instead, going for the Polerio. I think it has a higher level of reach. 

is the polerio defence an easier line to play?

I prefer it over fried liver stuff, and I am still learning but people who know what they are doing usually play Bd3. Qf3 can be tricky and I often forget that line. 

It's engine eval. Don't expect it to make sense.

On a unrelated note, white should not allow complications like that... Just grab the pawn and go!

Accept the Brilliant evaluation; they are very rare. Consider it a gift . . .

Yes this is a crazy line that leaves the king out in the open!

???

Well, you save your rook, castle and pin the knight in the same move. I think it is brilliant.

Castling on move 10 is a pretty brilliant move tbh. It's quite rare you castle into danger like that, but with the pin along the f-file, it's surprisingly safe for black.

Because it pins the knight, and it's just soo difficult to find if you don't know it, you have to see that the knight is pinned not to worry about the discoveries, and will still have to after unpin

https://www.chess.com/analysis?tab=analysis