Abdication

What the heck? You kill legions of undead, and after 2 days, Zoltan has 1000-2000 of each unit again....

And why, after obtaining the key in the Water Temple, the quest is marked as completed, but the gate doesn't open?

Regarding the first question, this is an artificial way to maintain the difficulty and danger posed by the AI. Unfortunately, the computer enemy cannot provide proper competition to the player without external help (a script). Therefore, when the main enemy heroes die, they are resurrected, and their army is replenished, becoming stronger each time. This:

a) serves as an incentive not to prolong the game and not to sit idle in the castle;
b) prevents boredom after the initial victories over enemy heroes.

Regarding the key, I don't remember the exact wording of the quest, but Markel said something like this:

"It would be good to find this key; it might be useful later."

The fact that the quest is completed is good; it will definitely help. How? It will become clear closer to the end of the game. There is no need for the main character to force his way through the gate right now; he has a different task at the moment.