Well, I can try to give a little more background which might help to address some of your concerns.
In the novel:
Caladan of course has a local population. They grow a kind of rice and export it. Caladan is considered a minor planet, but Duke Leto is well liked by the members of the Landsraad, the council of the great houses. By marrying one of the emperor's daughters (there are no sons), Leto could become very powerful... which the emperor wants to prevent.
Arrakis has several cities. The Harkonnen ruled from a larger one, Carthag, which seemed too difficult to defend in Leto's advisors' opinion.
The shields everyone uses render projectile weapons useless, only a slow blade or similar devices can penetrate them. Laser guns actually cause a minor nuclear blast when they hit a shield, which is why the imperial soldiers would be very careful where to use them... and certainly not against Duncan's ornithopter, which clearly had a shield.
Dr. Yueh was on the Duke's innermost council; it was considered impossible to break his conditioned loyalty. That is why he was able to shut down the shield.
The Atreides forces were stationed everywhere in the high surrounding cliffs and mountains, but the Harkonnens use an ancient, almost forgotten weapon against them, artillery. That way they cause rock slides and trap many of the atreides soldiers.
Oh, and the Atreides were very aware that the odds were not in their favor, so they attempted to form an alliance with both the Fremen (as potential fighters for their cause) and the local smugglers (in order to secure a way to leave the planet in case things got really bad)