The Witch's lab was the ancient dwelling for the ancient witch, Elizabeth Dexter, and was the location of her heart.
Carnby and Dexter's daughter had to find the lab in order to extract a strand of the heart in order to power up the blade in order to kill the witch.
The entrance to the lab itself was through two mechanical doors. During Carnaby's hallucinations from affected by the dagger's curse, Carnby was able to see the lab.
Despite the Witch living in ancient times, the fact that the entrance to the lab consisted of modern mechanical double doors seemed to be inconsistent with ancient times.