Set in an split timeline where an incident caused Aperture Science and Black Mesa to reverse roles; Black Mesa loses government support but becomes a highly successful and unethical private science corporation and Aperture Science becomes an underfunded government science branch suffering from its eclectic administration. The Black Mesa incident has not yet occurred in this reality, as the more capitalistic incarnation of Black Mesa have not ventured far into the world of interdimensional teleportation. Nor has the Aperture incident occurred, primarily due to a lack of government funding in AI technology. You are a corporate spy working for Aperture science, sent to sabotage the Black Mesa Upsilon facility and steal your competitors abandoned teleportation research, allowing Aperture to continue the interdimensional teleportation project...
Small typo. Aren't we living the 21st century? Half-Life plays sometime around the early 2000 after 1999. Other than that lovely advertisement poster. Not sure about the impenetrable sentence. I'd change it to something more scientific sounding.
Your sentences sounds like it would be fitting for fort Knox or a prison of sorts.
The timeline change occurs somewhere during the 20th century which is when Black Mesa became successful in this world so it's sort of retrospective. And I think impenetrable fits but I'll keep that in mind! Cheers!