Trap

Absolutely correct. The portal doesn't work. At first, I thought it was a clever design by the author, so I came back and tried again, even after clearing out all the enemies on this side of the portal – still the same. But I didn't give up, hoping for a more intricate solution. I finally managed to build the guild, and approached in the narrowest spot – it's impossible. Game version 3.1, map version 2.
It's very easy; it seems the game can't compensate for the lack of resources.