Tholian asteroid dock

The Tholian asteroid dock was a drydock facility constructed within an asteroid-moon that orbited a gas giant in the Vintaak system. It was there that the Tholian Assembly, in 2155, harbored the USS Defiant (NCC-1764), they unexpectedly obtained from the future, for study and evaluation. 

Unknown to the Assembly however, the Terran Empire became aware of the existence of the ship from the future, and sent forth the to capture the ship. Though managing to destroy the Enterprise in the ensuing battle, the Tholians were not able to prevent the capture of the Defiant, which, after coming under the control of a boarding party, managed to escape from their control, in the process inflicting damage to their facility. 

Background
Not identified in the episode, the term "Tholian asteroid dock", was coined by designer Doug Drexler.

Studio model
The in an asteroid embedded Tholian spacedock for "In a Mirror, Darkly", was designed and built as a CGI model by Doug Drexler. Drexler was inspired by the pre-production paintings, Ralph McQuarrie had done for the abandoned 1976-1977 Star Trek: Planet of the Titans-project. "I’ve always loved the idea of this, and was fascinated by Ralph McQuarrie’s TMP asteroid base painting.", Drexler stated. Using pre-existing CGI parts from various other models, Drexler constructed his model in one day, "I had to do a variety of ideas in a couple of days so an afternoon is about it. (...) Believe me, I had a kludge box full of parts and nernies. I couldn’t afford to build it all from scratch. There are a lot of freighter parts in there! " Drexler sent off his model to Eden FX, where CGI-modeler Mike Stetson expanded upon, and finetuned the model and where it was combined with the CGI-model of the USS Defiant (NCC-1764), built by Koji Kuramura and Robert Bonchune for their appearance in the episode. Both models were re-used to represent the same scene in the 2006 book Ships of the Line (page 29), and on the cover of the Star Trek: Enterprise Calendar (2006).

In regard to McQuarrie´s concept paintings, Rick Sternbach recalled, "In early 1978, I held that McQuarrie asteroid painting in my very own hands, as it had been stored in the art department over Stages 8/9 during the aborted first attempts to make a Trek movie. There were rolls of prints of Ken Adam’s stuff, and a couple of other McQuarrie renderings. The models of the triangular greebled Enterprise were there. The TOS set model was there. I could have walked off with the asteroid painting; it was such a little gem. So one weekend, I promised myself I would study all that material more closely come Monday morning. And it had all been spirited away. Well, some of it surfaced again some years later, and I seem to recall that Ralph got his little paintings back."