Interesting from Caste Thunder. - Under this caption the New York Herald publishes a letter from "A Yankee," who professes to be confined in Castle Thunder, in this city. The letter is dated "Castle Thunder, August 10th," and gives a heart-rending account of the writer's experience as a prisoner. We have for a long time been ignorant of the whereabouts of Baron Munchusen, but we have no hesitation in asserting that he is boarding with Captain Alexander, and writing letters to the New York Herald. We have no space for the Baron's latest effusion, but will give a specimen or two. For instance, speaking of the fare at Castle Thunder, he says, "generally we get nothing but horse and mule meat," and proceeds to remark that he has preserved many specimens of the mule bones, "intending, if the Union army ever releases us from this God-forsaken place, to exhibit them in the North." This is Yankee all over. No doubt the Baron calculates to make a fortune by the exhibition, which will be rendered doubly attractive by the lecture he proposes to deliver.

The Baron charges the commandant, Capt. Alexander, with assembling his officers, clerks, etc., at least once a week, in the officer's dining room, for a carousal. "The whole night is spent," he says, "in drinking, gambling, etc., in which women are allowed to participate. After each of these bacchanalian revels, if the Captain has been successful, he is all smiles and liberality; if he has been on the other hand, unfortunate - which it is usually his luck to be - he is as profane and brutal as a pirate, and orders half a dozen niggers flogged, and twice as many prisoners tied up by the thumbs, or bucked and gagged, and manages in the course in the course of a few hours to outrage every inmate of the Castle."