Since when did we call it a ‘castle’?
Why is the castle called a Rook?

This thread was the first time I saw somebody call a "rook" a "castle", I don't think any serious chess player does that (also there's "castling" already so why call a piece a castle). "tower", yes, but "castle", no.

it comes from Persian Rukh which is also where just about everything except for the bishop came from, why it looks like a castle tower I don't know

This thread was the first time I saw somebody call a "rook" a "castle", I don't think any serious chess player does that (also there's "castling" already so why call a piece a castle). "tower", yes, but "castle", no.
it's called castling because the rook looks like the tower of a castle

I know that, my point was to call the piece itself a castle as well wouldn't be very good for clarity.
Funny thing: while in English the piece is called "rook" and the special move "castling", in German (and quite similar in French) the piece is called a "tower" but the special move is called "Rochade" ("Roque") on the basis of the Persian name for the piece.
Simple. In ye olden days, the Rook was meant to represent a chariot, and the piece was designed that way as well. Up until the European version of Chess, this was a universal understanding. However, when the Ottomans and other Muslim peoples introduced their version of Chess (called Shatranj) to Europe via trade, they referred to the Rook by the Arabic name Rukh (as explained by V2U_Services). The Europeans simply Latinized the Arabic/Persian "Rukh" as "Rook", leading to the name we know today. As for the design of the piece, the sides of the chariot gradually evolved into the miniature archer's towers on the rook, and the wheels disappeared- the chariot evolved into a castle, something which the Europeans were more familiar with.
Let's check an etymological dictionary.
According to Wiktionary, the word rook is from "Persian رخ (rox), from Middle Persian lhw' (rox, “rook, castle (chess)”)."
A synonym is the word "castle".
Wiktionary indicates that it is possibly a cognate with Sanskrit रथ (ratha, “chariot”). The word for rook in Hindi is हाथी.
हाथी
1. elephant
2. (chess) rook or castle
Now let's look up the word castle
Noun
2. (chess) An instance of castling.
3. (chess, informal) A rook; a chess piece shaped like a castle tower.
So we can conclude that the word "rook" comes from Persian, and the word "castle" for rook is perfectly valid, albeit informal in English.
The chess piece's name, "Rook," is derived from the Persian word "rukh," meaning "chariot." The chariots of the early sets replaced the towers. The chariot was eventually replaced by a castle-like structure in European designs, perhaps as a result of misunderstandings. The modern castle-shaped rook that we are familiar with today is the result of this evolution.