Our resident King's Bounty expert, Tyson McCann, has taken Katauri's Crossworlds expansion pack for a spin in order to keep us informed via a thorough two-page review. Here's a snip about the new Orcs on the March campaign:
Aside from new units, there is a new building called the Military Academy placed on all the new large islands. This is a place where you can upgrade any of your troops for a cost. It also provides you with a class-specific quest that unlocks a special new skill once completed. Other new additions include a Wizard Tower that you can ascend via increasingly difficult battles to try for the reward at the top, lots of new standard and set items, a new Orc quest line, and a couple other more standard quests spread around.
The big reason to play Orcs on the March over Armored Princess is that the randomization has been improved, making new units available earlier and it also seems to provide access to the islands in a slightly faster way. A good thing, in my opinion. The only negative to the campaign might be to those who already own Armored Princess. It's really the same campaign you've played through once, only enhanced. If you feel like playing through it again (and the replayability is already quite high in this game), then it will be well worthwhile. If not, you'll have to settle for two minor, replayable campaigns and if it's up your alley, the editor.