Actually, it wasn't quite as secure a physical or virtual space as you imagine.
It was quite easy for anyone to get into the 9th Floor at 545 Tech Square simply by thumping on the door in the elevator lobby, because anyone on a Lisp Machine within earshot would just press Terminal-D to buzz open the door without getting up from their chair [1]. (And they could summon an elevator by pressing Terminal-E.) And many students and non-laboratory people (referred to as "random turists") knew the series of digits to tell a locksmith to make a key to those locks, or somebody who could make them a master key in the robot machine shop. And even if you didn't have your own key, there was always the MIT Lockpicking Guide. [2]
[1] http://dspace.mit.edu/bitstream/handle/1721.1/41180/AI_WP_23... (figure 5-1 and 5-2)
[2] https://www.lysator.liu.se/mit-guide/MITLockGuide.pdf
The physical locks on those doors certainly never stopped me from showing up unannounced, gaining physical access, and wrecking havoc by playing around with the Lisp Machines [3] and PDP-10's [4], spying on other people's sessions with the Knight TVs, printing out and Velobinding reams of documents on the Dover laser printer, and sleeping on the beanbag chair in the "Lounge Lizard Lispmacho" office.
[3] http://donhopkins.com/home/catalog/images/cadr.jpg
[4] http://donhopkins.com/home/catalog/images/mc-console.jpg
And then there was this thing called the ARPANET that you could use to log in without even knocking on any doors, picking any locks, or being physically present in Cambridge Massachusetts. Breaking into the ARPANET wasn't as difficult as depicted in The Americans "ARPANET" episode [5] (S02E07) where the KGB agents had to actually break into campus and murder somebody to gain access.
[5] https://www.youtube.com/watch?v=hVth6T3gMa0
The dial-up TIPs themselves actually had no passwords, and BBN would mail you a free copy of the "Users Guide to the Terminal IMP" [6] if you asked nicely -- including "APPENDIX A: HOST ADDRESSES". Then all you had to know was a phone number (301 948 3850 for example) and what to type (E, @O 134, :LOGIN RMS, RMS), and you were in.
[6] http://www.walden-family.com/dave/archive/bbn-tip-man.txt
You didn't even have to know RMS's password. If you tried to log in to MIT-AI with an unknown user name, it would ask you if you wanted to apply for an account, what your name is, and why you wanted to use the system, etc. If you answered sensibly (like "learning LISP"), you'd have your very own free off-hours "tourist" account [7] within days.
[7] http://www.art.net/~hopkins/Don/text/tourist-policy.html
Steven Levy wrote about the Hacker Ethic and MIT-AI Lab culture in his classic book, "Hackers: Heroes of the Computer Revolution" [8].
[8] https://murdercube.com/files/Computers/Heroes%20of%20the%20C...
The basic acquisition of every lock hacker was a master key. The proper master key would unlock the doors of a building, or a floor of a building. Even better than a master key was a grand-master key, sort of a master master-key; one of those babies could open perhaps two thirds of the doors on campus. Just like phone hacking, lock hacking required persistence and patience. So the hackers would go on late-night excursions, unscrewing and removing locks on doors. Then they would carefully dismantle the locks. Most locks could be opened by several different key combinations; so the hackers would take apart several locks in the same hallway to ascertain which combination they accepted in common. Then they would go about trying to make a key shaped in that particular combination.
It might be that the master key had to be made from special "blanks" unavailable to the general public. (This is often the case with high-security master keys, such as those used in defense work). This did not stop the hackers, because several of them had taken correspondence courses to qualify for locksmith certification; they were officially allowed to buy those restricted blank keys. Some keys were so high-security that even licensed locksmiths could not buy blanks for them; to duplicate those, the hackers would make midnight calls to the machine shop a corner work space on the ninth floor where a skilled metal craftsman named Bill Bennett worked by day on such material as robot arms. Working from scratch, several hackers made their own blanks in the machine shop.
The master key was more than a means to an end; it was a symbol of the hacker love of free access. At one point, the TMRC hackers even considered sending an MIT master key to every incoming freshman as a recruitment enticement. The master key was a magic sword to wave away evil. Evil, of course, was a locked door. Even if no tools were behind locked doors, the locks symbolized the power of bureaucracy, a power that would eventually be used to prevent full implementation of the Hacker Ethic. Bureaucracies were always threatened by people who wanted to know how things worked. Bureaucrats knew their survival depended on keeping people in ignorance, by using artificial means like locks to keep people under control. So when an administrator upped the ante in this war by installing a new lock, or purchasing a Class Two safe (government-certified for classified material), the hackers would immediately work to crack the lock, open the safe. In the latter case, they went to a super-ultra-techno surplus yard in Taunt-on, found a similar Class Two safe, took it back to the ninth floor, and opened it up with acetylene torches to find out how the locks and tumblers worked.
With all this lock hacking, the AI lab was an administrator's nightmare. Russ Noftsker knew; he was the administrator. He had arrived at Tech Square in 1965 with an engineering degree from the University of Mexico, an interest in artificial intelligence, and a friend who worked at Project MAC. He met Minsky, whose prime grad student-administrator, Dan Edwards, had just left the lab. Minsky, notoriously uninterested in administration, needed someone to handle the paperwork of the AI lab, which was eventually to split from Project MAC into a separate entity with its own government funding. So Marvin hired Noftsker, who in turn officially hired Greenblatt, Nelson, and Gosper as full-time hackers. Somehow, Noftsker had to keep this electronic circus in line with the values and policy of the Institute.
[...]
They went wherever they wanted, entering offices by traveling in the crawl space created by the low-hanging artificial ceiling, removing a ceiling tile, and dropping into their destinations commandos with pencil-pals in their shirt pockets. One hacker hurt his back one night when the ceiling collapsed and he fell into Minsky's office. But more often, the only evidence Noftsker would find was the occasional footprint on his wall. And, of course, sometimes he would enter his locked office and discover a hacker dozing on the sofa.
