While in this screenshot, Blizzard wants to make you believe that a bunch of Orcs are attacking a human base from behind, you'll notice that both the humans and the Orcs are of the same color, and hence are controlled by the same player. The fact that all the command icons on the bottom right are human, rather than Orcish further support this. Mostly likely, this map was created specifically to show off the Catapults ability to level trees.

From left to right, the human units on the top left corner consists of a Dwarven Mortar Team, an Archmage, a Knight and a Peasant. Meanwhile the Orc force is made up of a Grunt, a Raider, a Blademaster and the selected catapult. One thing worthy of note is that we can no longer see an Orc pushing the catapult, has Blizzard really taken it out. If so, it's rather disappointing, I really would have liked to see a slave pushing a heavy war machine around.

The human buildings on the top left include a farm and a townhall. There's also a goblin trading post on the bottom right. You'll also notice a chest the left of the Archmage, these chests can be retrieved by heroes and may contain either extra gold or useful items.