Why is a castle called a rook?

Why is a castle called a rook?can any one answer this, as a rook is a black bird what has this to do with castles.

from Persian رخ rokh, Sanskrit roth, meaning "chariot"

I also heard that rook was a boat

Rook is the name of a British rocket.

All very interesting but im none the wiser. 

The Rook has nothing to do with blackbirds.

There are several versions, but the one I find the most compelling is: 

The Indian pre-chess game, Chaturanga*, used a piece called the "rukh," that represented an elephantine war carriage used by the Indian army up until the 5th century. The The "rukhs" were actually the fortifications carried on the back of these elephants. As the piece evolved, the elephant itself faded away, leaving just the castle-shaped rukh, or Rook.

*Chatarunga means "four parts" and refers to the four parts of the Indian army: The boatmen, the cavalry, the elephant and the infantry.  

On a related note, the XiangQi (Chinese Chess) equivalent to the rook is a chariot (which is actually the same word in Chinese as the modern word for a car). Both Chess and XiangQi are derived from the same origin.

--Fromper 

Here's what I found with a bit of internet research:

"Originally, the rook symbolized a chariot. The Persian word rokh means chariot, and the corresponding pieces in Oriental chess games such as xiangqi and shogi have names meaning chariot. Persian War Chariots were heavily armoured, carrying a driver and at least one ranged-weapon bearer, such as an archer. The sides of the chariot were built to resemble fortified stone work, giving the impression of small, mobile buildings, causing terror on the battlefield. However, in the West, the rook is almost universally represented as a fortified tower. One possible explanation is that when the game was imported to Italy, the Persian rokh became the Italian word rocca, meaning fortress. Another possible explanation is that rooks represent siege towers. Rooks are usually made to look like small castles, and as a result, a rook is sometimes called a "castle", usually by non-players and those new to the game. This usage was common in the past ("The Rook, or Castle, is next in power to the Queen" —Howard Staunton, 1847) but today it is rarely, if ever, used in the literature or among players, except in reference to castling. (Here, "castle" is a verb referring to a move, not a noun referring to a piece.)"

 I don't know, but this explanation seems the most plausible... considering that chess is of Indian origin and Lord Krishna was Arjuna's chariot driver.

While we're on the subject of rooks and word origins, I recently heard (though I'm not sure if it's true) that the word rookie comes from the rook chess piece. The rookies in sports are the last ones to get a chance to participate, because they're new, just like the rooks tend to be the last pieces
to get involved in the action of a chess game.

--Fromper 

 Just like the knight, which is referred to as the horse.  Maybe the rook in military metaphor is the battle tank.  The queen is perhaps the Air Force, etc...

Really interesting thread!

Do you know anything about the origin of the term used for bishop?

In italian language the chess pieces are called quite differently respect to english:

- rook is the "Torre" ( = tower)

- knight is the "Cavallo" ( = horse)

- bishop is the "Alfiere" ( = "standard bearer")

For the first two pieces I found the italian terms more appropriate (the standard pieces really seems a tower and an horse) , for the third it is funny how different is the meaning... bishop (in italian "vescovo" a clerical hierarchy related term), "Alfiere" has a completely different meaning.

In any case my dictionary for the "Torre" word, related to chess, propose  both "rook" and "castle"

Rook comes from the Persian term Rukh meaning chariot as this was the piece in predecessor games of chess in India. These Indian chariets had large walled structures on them, more like a fortification. As it spread into Europe, the Italian term rocca (meaning fortress) may have caused the shape to change.

The elephant used in Chaturanga became the bishop. As nobody in Europe knew what an elephant was they picked different shapes for it (e.g. as it sat next to the king the Spanish turned it into a standard bearer, the French word for fool is similar to the word for elephant so it became a jester etc). With the tusks of the elephant pointing upwards it looked like the mitre of a bishop hence the name.

why is the knight not called horse? and bischop not pointi-haed-guy? and king not old man wiht beard?

The names of the pieces have changes according to the way chess has spread and travelled. Changes in the piece shape have also caused changes in the name.

The pawn obtained its name from Ancient Latin, as chess was played by the upper class who studied in Latin, the word for 'foot soldier' being peon (later to become paon in Ancient France).

The knight actually derives from cavalry, not a horse (although the original word for the piece was horse it represented cavalry), it was just easier to make the piece without the rider later on! It went through a simplification in shape before the current piece and in the middle ages they 'redid' the mount and rider shape. During this time the knight would ride into battle on horseback.

As stated above, when the bishop came to Erope the shape of the elephant tusks pointing upwards looked like a bishop's mitre. Religion and Christianity was extremely important in the middle ages and those high up in the church often avised the king. Seeing the piece with this shape and the fact that it was sitting next to the royal peces they names it the bishop.

The king would often lead the army in India and so was position with the pieces on the 'battlefield'. In chaturanga it was the Raja (the Indian king) and this would become the king in later forms - fairly straight forwards.

Also a Russian term for boat is called Rukh, or something like it. We've all seen how the chess sets coming from places without medieval, european 'castles' or 'towers' have rooks in the form of elephants, chariots, cannons, etc., & I think I've seen sets where the rooks are boats. Maybe worth a google image search.