PoV Forren
The effect of the sandstorm was mainly lowering the visibility of the field, but due to racial traits, this made his army weakened, so all I needed was to play the long game. I kept just one shaman inside the fortress and summoned as many troops as needed to keep the castle fully manned.
We were immune to the effects of the sandstorm; to us, it looked like a dark filter had been put over our eyes; it was nothing too big of a deal and didn't impair us as much. The strategy was clear: to abandon the castle and wait for the perfect ambush.
Of course maintaining a castle was no straightforward feat, so I had to summon plenty of units, but my goal was never to play such a defensive role. I summoned a bunch of assassins, and instead of ordering them to attack the other player, I ordered them to hide inside the castle, behind a corner, resting on top of a door, in whichever place the enemy had no direct sight of them.