Well, there's the 'mystery of Castiel's arrival' trope. Writers often play with the idea of how Castiel got to the Harry Potter universe and what his true purpose is there. Also, the 'cultural clash' trope is popular. Since Castiel comes from a very different kind of supernatural world compared to Harry Potter's, there are a lot of interesting interactions based on their different beliefs and ways of doing things. It makes for some really engaging stories as the characters learn from each other and grow.
One common trope is the 'fish out of water' situation where Castiel is suddenly in the Harry Potter world and has to figure out how things work. Another is the 'powerful ally' trope where he becomes a crucial help to Harry and his friends.
Chapter 663, "How Did These Two Start Fighting?!", is the clearest early answer because the buildup finally cashes out in a real clash. The chapter gives readers a real answer by opening on Jiang Shixu received the message in daylight the next day.
Chapter 614, "Storm Crisis", is the clearest early answer because it turns Storm Crisis Wilderness's situation into a concrete plot event. The chapter turns that into something concrete by moving from Today, the wind on the island was stronger, and for safety reasons,...
Chapter 649, "Where Did the Wild Wolf Come From?!", is the clearest early answer because it turns Yu Li's situation into a concrete plot event. It opens with Several people successively lay down in the tent, and before long,...
Chapter 405, "The Person Saved Casually Turns Out to Be a Diplomat?!", is the clearest early answer because it turns Foreign Liaison Committee's situation into a concrete plot event. Early on, The medical student was completely dumbfounded..
Chapter 334, "Do you not want to because you look down on me?", is the clearest early answer because it turns Do's situation into a concrete plot event.
Chapter 435, "A Skilled Housewife Can't Cook Without Rice", is the clearest early answer because the violence around Dragon Ridge Island becomes final instead of hypothetical. The chapter gives readers a real answer by opening on The elderly dean looked at Zhao Lang with a tinge of regret.
Chapter 271, "How Did Shen Lan Marry an Old Fool?!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from He had barely finished speaking when he received a withering look...
Chapter 570, "A Simple Auto-Attack Baits Out an Ultimate from the Enemy", is the clearest early answer because the buildup finally cashes out in a real clash. A useful checkpoint is the way the scene starts with What the heck is that?" "??
Chapter 314, "The teacher will surely cultivate you into a new music star!", is the clearest early answer because it turns Zhou Wenshan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with After chatting with Zhou Wenshu for a bit, Jiang Li called Jiang...
Chapter 360, "There's No Shame in Wearing a Dress Designed by Your Mother", is the clearest early answer because it turns Younger President Jin's situation into a concrete plot event. It opens with Jiang Li showed no fluctuations in emotion and remained unperturbed...