You should unlock new items to build right after the first mission.
Do you remember which map didn't allow you to properly manipulate the horde ?
Both of the ones I tried, the scrapyard and military base, I just got creamed by the horde trying to get out.
I've tweaked the first maps to have more fire spots, which should give you more possibilities to manipulate the horde.
Also, the map generation now avoids placing objectives too close to the spawn points.
