What's compelling about Terry Pratchett's Guards! Guards! is the way he turns the classic fantasy narrative of the one true king on its head. There are certain expectations in the fantasy genre of the rightful king in exile. He's got to have an ancestral sword or a birthmark or something by which he can be identified. He's got to have some secret explanation as to how he came to turn up claiming his legitimate rule. He's got to prove himself in some remarkable act of valor.
In the Lord of the Rings, there's Aragorn. In the Song of Ice and Fire, there's a major spoiler for those who haven't finished the most current season of Game of Thrones. In The Once and Future King, there's Arthur.
When you find this character in the story, you generally know that somehow he should come to power, and he will work hard to keep the people in his kingdom safe and happy.
This is not really how Discworld works, though the series generally celebrates these high fantasy ideas as much as it makes fun of them. Guards! Guards! is one of the strongest examples of this.
Guards! Guards! is the first book that covers the Ankh-Morpork city watch as operated by Samuel Vimes, one of my favorite Pratchett characters. Vimes is heavily involved in the series as a whole, and he is as developed and pragmatic as any fantasy character Game of Thrones could throw at you. He seems to be more influenced by film noir detective stories than by high fantasy tropes.
The rest of the watch is equally colorful. Corporal Nobby Nobbs, Sgt Colon, and Carrot Ironfounderson are unforgettable characters even in the Discworld with its great wealth of well developed characters.
While this isn't my favorite Discworld novel, I would consider it an essential read for people who like high fantasy.