I put spearmen because it doesn't make sense to put ballistae (we're talking about one castle; if there are many castles, then ballistae are used there), as it's very difficult to drag them around with you at the beginning of the game... Although, it's a very powerful weapon, no doubt. If you do build them, you need to bring them to decisive battles and castle sieges. Although they can also be used for defense...
I used to always build monks, now I build crusaders. If I have many castles, I build both in a ratio of approximately 4:6 for 10 castles. Although, both are good.
I'm used to building champions. Because they help to get out of not very serious battles without losses, thanks to their first strike.