I seriously doubt the game would let you play as the monsters on a whim like that. There's no support anywhere in what we've seen here, or what we've seen in any game, that would encourage us to think that we can be the monsters.
Also, unless the AI is pretty damned advanced it'd be boring.
Keep in mind that the teaser images aren't even screenshots, and are meant to give a very basic overview. That's like if a DF teaser showed only a mason and axelord, and the viewer assumed "Oh, all dwarves only wield axes." The teaser is meant to show the basics of gameplay and give a broad overview of "these are things that may happen" while the actual game may include any amount of detail that's more complex and too heavy to show in the teaser.
On the other hand, while assuming that you can only play as dwarves wouldn't be totally accurate, assuming that the game's completely focused on and built around them would be.
Of course, to be honest I'd be surprised if the finished game looked anything like what they have in mind right now, so whether they've got different creature appearance support planned might not be all that important.
My guess is monster spawners you have to take out, ala Gauntlet. There's some on each island and they constantly just puke out guys at varying rates. Or maybe they just wander in from the edges of the island where you aren't.
I doubt that. First of all, that'd make the spawn rate the same when you first started out as when you were fully entrenched.
Secondly, that'd make the goal of the game to defend your castle until such time as you could be bothered to go smash the spawner. Assuming they spawn in waves, that might be fairly easy to pull off.
Third, uninhabited islands would become absolutely infested with resource-draining critters for no benefit, or else you'd need some sort of die-off mechanic for just that one thing, and it'd still tax your system more than was necessary.
This title is just an inkling of something, if that. It's way too early to anticipate.
It's never too early to wildly and idealistically speculate, though.