There are different upgrade strategies for storming a castle and battling in the field!

You've said it all yourself. We're not playing a game of "who can level up their hero best for a field battle." If you want to win, you need to think about how you'll take your opponent in the city or gain an advantage that will make staying in the city pointless (for example, having a strong secondary hero or a second strong hero who can control the map while the first one locks down the opponent, etc.). Otherwise, you can just level up a pure warrior and not worry about it...