If you've played the original Empire at War, then you know that you couldn't move from planet to planet without fighting for control of them. Tyber, however, is able to slip under the radar by sending his agents to "corrupt" a planet. Once it's corrupted, you can travel past it freely by paying a bribe or - better yet - get an advance look at where enemy units are stationed on its surface. If they look too intimidating, you'll be able to send in a saboteur to soften up their defenses a little before battle.
Once you've taken control of a world, you can choose the placement of your own forces on a static map - something the original Empire at War didn't let you do. You'll have 40 new units to place, including new ones for the Imperial and Rebel factions (sadly, only the Underworld gets new single-player missions).
But while playing with the Empire's Force-enhanced Dark Troopers sounds fun - especially if you've bribed them to your side - the Underworld's units let you really fight dirty. One of the new attacks, for example, is to infect enemies with poison gas; when they die, they'll turn into green toxic blobs that can harm other units. Incredibly evil robot bounty hunter IG-88 - playable here as a unit - uses this quite a bit, contaminating whole areas with radioactive sludge.
For a real guilty pleasure, though, we'll take the Saboteur, who can place "sticky bombs" on enemy units. These can be detonated at any time, turning enemies into unwitting suicide bombers.
