I wanted to know what is the difference between 濠 (gō) and 堀 (hori). I've only seen 濠 in context of 環濠 (kangō), 豪華な (gōkana) and 豪州 (gōshū). From what I know 環濠 and 堀 both means moat. Well, then, what does 濠 means alone?
Etymologically, in Chinese, 濠 or 壕 (depending on whether it is watered or dry) is the character that represents the word háo "moat, trench". 堀 is a now rare variant of 窟, which stands for the word kū "cave, burrow, hole", and does not have the same meaning as the former.
However, in Japanese, the notion "moat" is represented by the word ほり, which obviously derives from 掘【ほ】る "dig, bore", as if literally saying "dug" or "digging". Consequently it is felt more natural for Japanese to employ the character 堀, which looks similar to 掘* and has radical 土 associated with "building", when writing this word. It is an example of 国訓, a Japanese-only meaning the Chinese language has never given to the character.
Although assigned the kun'yomi ほり as well, 濠 is far from the primary option for representing the native word. Instead, it is used as a synonym to build on'yomi compounds such as 環濠 (or 防空壕, 塹壕 via 壕). This is just contrary to 堀, which seldom appears in on'yomi クツ it technically has.
* Unlike Japanese, 掘 and 堀 are not homophones in Chinese (jué < *gjwot vs kū < *khwot), though they are presumably cognate.
堀 refers to artificial ditches in general. 壕 is a dry version, and 濠 is a wet version filled with water. Practically, I kind of feel 壕 and 濠 tend to be used in military contexts, whereas 堀 tends to refer to permanent and beautiful ones surrounding large castles.
Note that 壕 and 濠 are phono-sementic kanji; the left parts (土 = soil and 氵 = water) contribute to the meaning, but the right part, 豪, just represents the sound ごう. The kanji 豪 on its own means something like "great" or "magnificent", as in 豪華, but its meaning is not important in 濠 and 壕. 豪 in 豪州 is just an ateji.