Strange castling rules.

Since I started playing, I followed a certain rule about castling.
When there is a opponent's piece aiming down the path between king and rook, I believed that castling is not allowed.

But here the rule I know of doesn't seem to be followed.
My opponent castled while my queen was aiming down the path.
So castling is universal?
I mean can I castle whenever I like?

Look at my game, my opponent castled at 25th move.
http://en.lichess.org/IU21snUH/black#24

Castling on queenside is allowed when the b square (the square the rook passes over but the king does not) is attacked.

Castling is *not* allowed if:
*Either the king or the rook have already moved.
*You are in check.
*You would move into check.
*The king would pass over a square where he would have been in check.

#2, yup!