Fang was extremely excited, not only because he was going to feel such a god-like feeling again before leaving, but because there was another battle waiting for them at the surface.
As the two got the plan from Sloth, they waited for his signal. The first part of the plan was to mobilise the extra and unneeded masters from here first.
It wasn't just to empty enough space for others to slowly and orderly retreat, but they now needed to go up there and start fortifying the entire place with traps and formations.
Building defences out there wasn't feasible. The retreat plan would take roughly one day, and they wouldn't be able to build anything of value during this short time.