You can set any resolution manually in the Resolution field. The optimal minimum vertical resolution is 684 (if it's lower, the resource panel will be hidden in battle).

So what? What's the point if the game doesn't launch in 864x684x16 resolution (original)? The reasons are listed above.
By the way, I noticed a bug: when playing with an ally, you can enter their castle, but you can't buy units there. In the HD mod, using the "buy all" button (click on the castle at the bottom), you can buy all the units in your ally's city during your turn, which, for example, can allow your ally to defend themselves with this out-of-turn purchase.