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Don't forget, give your prospectors an Ornithopter. That way, they move faster.

As for attacking Harkonnen fortresses, tactic is simple.

1)Send Spies.
2)Ascertain level of Weapons.
3)Attack with bigger no. of troops and [IMPORTANT] make sure u have the same weapons as the Harkonnen garrison, i.e if they have nukes, make sure all ur troops have them.

Secondary Tip : Ride on a sandworm and head to the battlefield, ur troops will get a huge (!) morale boost.