- Замок (Castle)

What's the problem with upgrading horses? Just go to the stable and it's done.

There really aren't any problems with upgrading horses. The problem might arise with building them – after all, you need 20 wood just for the hippodrome. And another 10 for the stable, without which the hippodrome can't be built.
And you don't always find that much wood.
Besides, the tactic with Tyris and cavalry has a significant drawback – you have to play very carefully, because an unjustified loss of even 1 cavalry unit in the first 1-2 weeks is not a pleasant thing.