Guys, help me out. It's the 5th campaign, for the Elves, the third mission (defense). At the beginning, everything is clear – kill everyone. But what to do next? Should I just sit and guard my Castle, which they constantly attack, or should I move around the map and save the hero? The task says to hold the Castle and save the wandering hero. If you sit in the Castle, you won't have enough resources; if you go to save the hero, they'll take the Castle. If you stay in the Castle, there are no resources to develop, and you still need to save the hero. What should I do? That's the question.

You're wrong. You have enough resources for everything.
Defend the Castle (it's easy), kill a few enemies (they'll come quickly). Then, leave the Druids with the trees and run to take the Elven Castle on the other side of the bay. Castle, money, resources – do it quickly. And run back, taking troops from the new Castle (don't upgrade Dragons – it costs a lot of money). When passing by your first Castle, summon troops and start destroying the Necromancer garrisons and push into their territory. (Don't forget to reinforce your first Castle and leave troops there). DO NOT EVEN THINK ABOUT TOUCHING THE DUNGEONS, otherwise the mission will freeze. Kill the Necromancers and run back to your first Castle. As soon as the city is fully built (COMPLETELY), place troops in the garrison (inside the city! I hope you understand, :)). And if the task is not completed (Reinforce the garrison), then gather troops from both Castles. As soon as you have enough troops, take everything that's more than the garrison and go to rescue your friends (destroy the dungeons). But the "Reinforce the garrison" checkbox must be checked. You free the heroes, and that's it. I never got the "Hold the city" checkbox.
That's roughly it. ;)
Go for it.