BTW: I'm playing on 'Accessible' difficulty (2/7).
The second level was more challenging, an early zombie attack forced me to start over and focus on building walls. The goals for the level seemed a bit ridiculous, though, it forced me to almost cover the entire map in Tents to achieve a population of 600.
The Swarms are blocking my progress, and they seem to difficult to be realistically beatable. The resource used to defend against them is Empire Points, which you can find in the tactical missions. So I went out and got some more of those, and then the Swarm was easy.
I tried The Coast of Bones, but was overwhelmed by a wave of zombies from the east. I was wondering if you could replay missions to grind research points, but according to a post by the developer, this is not possible, replaying missions don't give you anything.