Playing with fewer than four players and without mercenaries is not intended by the rules and will upset game balance
The main problem is the victory condition which likely is achieved too easily. As the goal for each player is to capture seven Crowns (primarily by taking cities), fewer players means more Crowns per player without any territorial struggle which upsets the game balance. With each player being closer to victory by default, a single city changing hands might end the game, quickly and without much of a build-up to that point. Maybe with two players it can even be won without ever facing your enemy.
Another concern is resource distribution. The balance here might be upset by fewer players as well. I don't remember that part of the game well enough though.
The mercenaries are introduced to balance the lack of player-controlled powers. It is not marked as an optional variant, but is the intended way to play with fewer than four players.
If you really want to play with two players and without mercenaries, my first advice would be to adjust the number of Crowns required for victory and maybe remove some of the Crown Cards, as well. This will probably require some experimentation and a different target number for two and three players, respectively, but it will help with some of the issues. Keep track of how other elements of the game like income or cities' special abilities pan out as well, and if you feel like something is consistently off, try adjusting it accordingly.