It's a mock oath ascribed to sailors, though it appears to be a comic embellishment of a slightly different oath, my timbers . Apparently Mr. Marryat invented the phrase with an eye toward avoiding his readers taking offense at stronger words. Hence, when timbers are shivered, they are broken into splinters. A curiously similar word is shake , a fissure that forms in wood while it is still growing. The phrase shiver my timbers was purportedly adopted later by cricket to refer to the scattering of wickets.
Castle derives from Latin castellum.
The easiest way to answer your question is to start from the bottom and move up. Some other relatives of chaste are caste , castigate , and incest . You might be surprised to learn that deer originally referred to all beasts, or at least to quadrupeds. That restriction began in the early middle ages and was firmly entrenched by the end of that period. It originally applied to the flesh of any game, but by the 17th century it began its shift to refer solely to deer.