Tricky chess puzzle (part 2)

Yes Guys! I brought you an other tricky chess puzzle for stretch your brain! 

Can you solve that puzzle on the picture? Yeah, that's a MATE IN 1! For white...

Yeah? You did? Or you just don't understand it?

Well, let's see!

So, we can give black a check with the rook, but the opponent's king simly captures it:

That's just a lost piece.

Otherwise we can move with  our king and give him check:

However it won't be mate. #

Then what can we do?

So guys! Have I ever said that the white rook on a1 and the white king have moved before?

Soooo... They can do the long castle LEGALLY!  

Just becouse it's the endgame it doesn't mean that the king and the rook have moved before. It can be real!

So you see, the answer is:

The castle....

And the black king is DONE!

Did you enjoy this challenge?

Tell me in the comments!