Toronto's City Hall has been used twice in the Star Trek series. What was it considered as being?

https://en.wikipedia.org/wiki/Toronto_City_Hall

According to this it has only been used once in the episode "All Our Yesterdays", and it only had to have been used during the remastering of the series as the crew never left Hollywood (or the surrounding area) to film any episodes of the original series. Without seeing the episode there is no definitive corroboration of this fact.

trivia answer is
destination of an alien portal

In the episode...the alien portal was a doorway, an Ice cliff and a brick wall that were all constructed in the studio on the lot of Desilu Productions where Star Trek was filmed. If you are discussing the exterior shot of the "library" in the episode, then there is no corroboration that I have seen as of yet. But it is not the "alien portal" that is being displayed here as the answer, and that answer is too vague.

Toronto City Hall used as Alien Portal:

1. In the Star Trek: The Original Series episode "All Our Yesterdays".
2. The city hall was seen in the Star Trek: The Next Generation episode "Contagion" as one of the possible destinations of an alien portal.

https://en.wikipedia.org/wiki/Toronto_City_Hall

The question dealt with Star Trek...not Star Trek: The Next Generation. So, I stand that the "portals" were constructed on the Desilu Lot. (as Star Trek did not have enough money to go "on location"...especially out of the Country to Canada and on the East Coast of the Country.

The IMDB site makes no reference to Canada or City Hall. And without seeing the remastered version (which is what I believe that pmac41 is referring to, having known that they never used exterior shots of real buildings in the 1969 filming of the episodes. All backdrops were created by artists). So, I will say that barring all other possibilities, the correct answer would still be "the library." But without seeing the episode, this cannot be corroborated. The Star Trek official site, makes no mention of Toronto at all.

However, again without corroboration, it is possible that the exterior shot of the City Hall could have been used as a destination through the portal at the library, but not the portal itself. Or a revised backdrop that you were able to see the building through the portal at the library.