When you're inside the building, the notch closes behind you, and the
inside of the building starts glowing slightly red, though the red
runs through the strangest changing patterns you've ever seen (and
you've been through a lot!)
The tunnel continues in front of you, splitting into two branches, the
walls, ceiling and floor lit by that strange crisscrossing red
pattern.
The tug on you stops, and you emerge before a tall black cross-shaped
building, dotted with myraid patterns, notches, grooves, heat and
light and high-pitched sounds. You step forward, and as if by magic of
your arrival, one notch in the building opens wide, about eight feet
up, allowing you to slip in easily.