"In honor of our fathers" - "One Khan, One Clan" [Campaign 3, Mission 2]

I'm also having trouble with the Walkthrough. It started when the Purple hero appeared. My tactics didn't work. By the time that elf (and not just one) arrived, the Turquoise hero had already captured one Castle. I gathered an army, went after the Purple heroes who were blocking the way to the sea. I defeated one, and ran to the Castle for reinforcements... But!!! Then, the Turquoise hero jumped out of a portal and attacked me! He completely crushed me!
The main problem is gold. I don't have enough money for an army, and not enough time to accumulate it! I captured all the mines and quarries I could, all the artifacts I could, etc.
Please advise on how to proceed! I've already started the game from the beginning four times....
To be more specific, I have the following questions:
1. How to develop a Castle?
Is it more important to increase the army or defend the city?
2. After capturing the brown Castle, should I clear the territory or go to the Cyclops?
And which hero should I use to go to the Cyclops? And how to develop a second Castle?
3. After the Purple hero appears, should I break through with a large army to the green Castle or destroy his two ships?
4. Finally, when hiring units, which ones should I focus on? Centaurs and Wyverns?
Thank you in advance!