Archibald, mission 7.

Option 2
1. On foot. Generate a Barbarian in the tavern who quickly levels up Logistics and Pathfinder, gather Ogres (preferably generated and upgraded) around the castle, and set off. Minimum - 6 days
2. Using a Dungeon Demon. Generate a Wizard in the tavern, and a Dungeon Demon at level 5 in the yellow castle. Gather Ogres, capture the yellow castle, build magic, and go. Minimum - 5 days
You can also generate a Dungeon Demon in the pyramid, but I couldn't find a hero who would level up Navigation, Expert Wisdom, and Pathfinder, while also gaining Knowledge skills.