Ambassador

Tell me, is it necessary to have 200 archers to capture Horncrest?

Yes, and you need to buy a catapult in the workshop. I don't think it's difficult to gather 200 archers, as besides the 2 archer towers and training in the city, there are also 2 groups of archers that will join you.

Added 1 minute ago

I'll write a detailed report later, but for now, a question: what will happen if I kill Duncan and the rest of the gang?

I'm waiting.
If you kill Duncan, nothing will happen. But if you kill someone else (Ilya, Wolfsten, Isabel, Frida), they will reappear near the city.