The gate looks there to me. It's the same as the other identical enclosure next to it, the gate is the spot on the west side, yes? The spider could just head out the gate. It apparently chose not to, and that's why it is now a flaming corpse.
Nice shooting, BTW. You managed to somehow set just the spider on fire without setting anything else on fire.
As for how intended it is, well, it's a very random mapgen, and sometimes accessibility of areas can be rather impaired. ONE spider unable to zerg you probably isn't a big deal. Not that I've ever encountered monster hunts where enemies swarm effectively. They tend to just get confused and mill around in aimless circles going nowhere in particular.
Actually there were two spiders stuck, I just took a screenshot of the alive second one. The first I used flaming arrows on. I was unable to directly reach the position directly adjacent to the spider, meaning I couldn't squeeze between the barrels for whatever reason.
As for getting swarmed, it can happen depending on where craft is in relationship to most the monsters. They usually behave a bit smarter if you're out in the open as well.