La Louvre
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I tried several different approaches. First, I captured the Castle, lost half my army, and Laslo showed up and gave me a beating; even the walls couldn't stop him. Then, I decided to attack Laslo directly, met him on the road to his second Castle – the result was the same. Although I defeated the garrison with undead and dark elves without any losses, and I barely lost any troops in the battles for the mines, plus I had dwarven artillery, but it didn't help me much.
In the end, after defeating the garrison, I decided to retreat to my Castle, and I sent Duncan to some remote corner. Laslo entered my territory and headed towards Duncan, as he had a small army. But Duncan has a portal to the city, and as soon as Laslo was within a day's march of Duncan, I sent him to the city, and then I chased him back into a corner several times. I bought reinforcements twice during the week. Then Laslo got tired of chasing Duncan and ran towards the city. I already had a significant army, the city was fully fortified, plus an additional detachment of 700 mountain defenders. In short, I defeated Laslo without any problems, and then I gathered a small army and captured the city.