This is the famous Winchester factory that you have probably have heard about. More than likely it might be you've heard about it at least because of Mrs. Winchester who was possibly schizophrenic and moved herself as far away as she could from her husbands company, built a house, and didn't stop building until the day she died. It was because of the dead men/women/children who were talking to her because they died by the guns her husbands company produced.
This facility originally was city blocks long/wide and was a major economic boost for the city of New Haven. The Winchester company was one of the top armament manufacturers in the late 19th/early 20th century. This facility predates the civil war. Most of the original facility is still standing, however most of those buildings are now owned primarily by Yale and have been gutted and renovated for various purposes, such as a freaking parking garage, because we always need those.... ##:( The others were torn down and newer buildings put in its place.
The area which is still unused and abandoned spans an entire city block long. Recently however Yale has started renovating the last original buildings left of this most infamous factory.
It is still accessible, though now it has gone from completely abandoned to partially active due to the construction. You just need to watch out for construction crews who work during the week though I have never had a problem with them. They have not gutted all of the buildings yet. The city block consists of buildings that are all connected in some fashion either through air walkways, be careful not to fall through holes please it would be a 4/5 story drop, or by hallways connecting each floor. Each building is at least 4 stories tall, some 5 or 6.
Since heavy duty chemicals were used to create the guns/rifles and the bullets I would recommend wearing a respirator if you go.
Here's some more historical info about Winchester:
http://nhpt.org/index.php/site/district/winchester_repeating_arms_company_historic_district/ 
Author