[xboxnolifes]

In some ways it is a part of gameplay, since knights are the only piece that can travel through other pieces. One one hand, you could just say that there are additional movement rules pertaining to either just knights or to all other pieces besides them. On the other you could also say that a chess board is actually only a 2-dimensional representation of the game state and that you actually play in a 1-dimensional plane with more advanced movement rules.

[V-2]

Depends on your definition. King and Rook sort of travel through eachother when castling.

[c22]

Technically the king jumps the rook.

[V-2]

How do you know it's not vice versa? :) Both pieces move. And when you castle on a real board, you're supposed to move the King first, then the Rook, so it is indeed vice versa, the King doesn't have to "jump".

Besides, note I responded to a comment that stated "knights are the only piece that can travel through other pieces".

So if we already conceptualize Knights as "travelling through" rather than jumping over...

[c22]

I know because castling developed from the King's Leap [0] which came about as the game evolved to faster more exciting play. By allowing the Queen and Bishops to move an arbitrary number of spaces various mechanisms were tried for protecting the King revolving around moving multiple spaces or jumping pieces. A common sequence became to move one's Rook adjacent to one's King, then using the next move to "jump" the King to safety. This eventually became formalized as modern castling and can be seen by noticing how the pieces move when castling kingside vs queenside.

[0] http://history.chess.free.fr/mediaeval-chess.htm