We are approaching the holiday of Purim, where the megillah is read in most cities on the 14th of Adar, but in walled cities it is read on the 15th. One of the terms for a walled city is krach כרך.
Until very recently, I would have told you that the origin of that word was fairly obvious. The root כרך means to "bind, wrap, surround." The binding of a book is kricha כריכה, and so an individual volume in a series of books is kerech כרך. In the Pesach seder we read about how Hillel would wrap his matza (clearly not the hard matza eaten by most Ashkenazi Jews today), maror and the sacrificial meat. That wrapping - which today we duplicate by eating matza and maror together - is called korech כורך. From here we get the official word for sandwich in modern Hebrew - karich כריך.
So I assumed (as Klein writes) that a krach is so called because the walls surround it. However, I recently started studying the Talmudic tractate Megilah, and on the first page, Steinsaltz notes that a krakh is a large, generally walled city. He mentions the theory that it derives from the root כרך as we mentioned before, but only as the second possibility. The first possibility I had never heard before.
This theory (also brought first by Even Shoshan) says that krach derives from the Greek charax, meaning a fortification - a location defended by reinforcing walls. There were many places known as Charax in the Greek world, some of which can be found here. The word charax originally meant the pointed stake used to make the walls, and later came to mean the fortification itself. In this way it is very similar to the word palisade, which means a "a fence of wooden stakes forming an enclosure" but originally meant the stake itself. In fact, many translate charax, and even krach, as palisade. (A synonym of palisade is rampart, and after looking that up I now know what that line meant in the Star Spangled Banner.)
A couple of familiar English words are cognate with charax. One that sounds similar, but with an unusual etymology is "character":
mid-14c., carecter, "symbol marked or branded on the body;" mid-15c., "symbol or drawing used in sorcery," from Old French caratere "feature, character" (13c., Modern French caractère), from Latin character, from Greek kharakter "engraved mark," also "symbol or imprint on the soul," also "instrument for marking," from kharassein "to engrave," from kharax "pointed stake," from PIE root *gher- (4) "to scrape, scratch." Meaning extended in ancient times by metaphor to "a defining quality."
A less obvious cognate is "gash":
1540s, alteration of Middle English garce "a gash, cut, wound, incision" (early 13c.), from Old North French garser "to scarify, cut, slash" (Old French *garse), apparently from Vulgar Latin *charassare, from Greek kharassein "engrave, sharpen, carve, cut," from PIE *gher- (4) "to scrape, scratch"
In this case, the foreign etymology of krach seems more convincing to me, although I'm sure most Hebrew speakers would find it difficult to believe. Perhaps to educate them, we can start calling the stake-like toothpick or skewer (used to hold a karich together) a karach?