Take over the Tradint Post and build a second mine nearby. While your base is expanding, you can send your troops to have some fun: storm the city (1) and destroy dwo Dark Spires (2). After a while the enemy will start to attack you from the North - nothing serious, really.
Try to expand your territory to the North-East (3), taking over new mines. The enemy will react aggressively if you storm his city (4) and will probably attack you with most of his troops. Think it over, then.
If you secure your positions in all three cities, you'll have to destroy the three Dark Spires that are left (5). It's easiest to do that with air forces (Rukhs!). Don't waste time on destroying other buildings.
HINT: There's a sub-quest called "Find the Scientist" in this mission. He's in (6), and you'll have to lead one of your heroes to him. He'll then tell you to take over a Salamander Nest (7) and lead one of these beasts to him. You'll get a telescope for that. He'll then ask you to find his lost Clockwork Man (8). Bring it back to him using a Magic Whirlwind.
HINT: You'll get another hero after this quest, Andromolek, who will enable you to build Dark Spires and train Dark Alin units for the rest of the campaign.