Knights is a tricky card, and not easy to formulate generic rules about it.
I'll try some, but do know that every rule here has its exceptions.
0. In general, Knights aren't a great card. Start with 'no' until you find reasons to go 'yes'.
1. Knights trash cards from your opponent. If it's hard for your opponent to get cards, they're good (say a kingdom with an engine but no +buy)
2. Knights can trash cards where the split is very important (say, a game with Duke where trashing a few Duchies makes a huge difference)
3. Knights can be the only card that provides something the kingdom is lacking. Again, think about a strong engine without +buy (Sir Martin), or without trashing (Dame Anna), or villages+coucilroom finished off by Sir Michael.
4. Knights can be really good if you can select what you trash (scrying pool, oracle, spy, ...)
5. If both players go Knights, it's usually good to get as many as you can get. However, if you're the only one buying them, you should probably stop after getting 3-4.
You asked for a kingdom so here is one:
Village, Smithy, Farming Village, Rabble, Trading Post, Border Village, Knights, Harem, Duke, Silk Road.
Here is a game that shows what can happen if you ignore knights where you shouldn't.