Even when they tried to activate the trap formations laid under the ground prematurely, they failed.
These formations were now linked to the colossal cloud of golden spirit power. Such connection was draining spirit power from all the formations underground and also protecting them from responding to any outside stimulus.
Failing this ended up making them more troubled and worried. They slowly started to grow scared. They built this trap based on this special area here, over the special underground tiles they found in this pocket world by accident.
It was as William guessed before, they didn't build anything. They found this place, studied it out of curiosity, and ended up learning how it was great in setting a deadly trap for their enemies.