Worshipper – “Conquest” [Campaign 2, Mission 3]

What exactly is the problem? Even without neutrals, you can manage with two castles, and to increase the chance of joining, keep unupgraded archers in your army as well; there are many of them on the map, and they pose very little danger to your troops.

Added 2 minutes later

I completed this mission a long time ago, but I think 9 months is too long. I didn't waste any time and completed the mission.

Yes, you immediately take diplomacy, wait for the first unit to join, and then it's off to the races – 120 archers, 500 fairies, and 250 dancers per day.