Star Wars: Empire at War/Imperial units

The Imperials wield AT-ATs and Star Destroyers that spreads fear across the Galaxy. If you do the Imperial campaign expect the Empire to be harder than the Rebels like expect to fight to many rebel heavy units that can team up and destroy walkers. Also X-Wings can take down even all spaceship or cruiser of any kind even Star Destroyers.

Ground Forces
Expect the Imperial ground force to rival the Rebels units.

AT-ATs are the only thing that can take on all Rebel Ground tanks but watch out for SnowSpeeders which can take down the monsterous walkers.

Also whats worst is Darth Vader or his master on the battlefield will cause chaos to the Rebel forces as well(If Hero units like Ben Kenobi get into a lightsaber duel with Vader your other units will not interfere with the battle and ignore it).

Starships
Imperial starships are powerful foes and Star Destroyers especially admiral pietts star destroyer is the worst foe for the Rebels. Also the Starfighters are all the same thing called TIE Aircraft.

Hero units
Darth Vader and his master are sith lords which wreck havoc on even all units but Red Squadron and X-wings can shoot donw Vaders Tie if he ever meet them in dogfights.

Also the Death Star is somewhat available.