I was on this random Wikipedia page and saw something cool.
"It was a secret town in the Soviet Union until President Boris Yeltsin decreed in 1992 that such places could use their historical names."
I've never heard of a secret town before so I did some Googling and this article is all I could find.
I would love to more know about secret towns. Did other countries have them? How did they keep them secret?
The Russian "secret towns" were actually called "closed towns", so googling that may give you more results. In Russian they are called "3ATO" which stands for "closed administrative-territorial formation". Primary sources of information would be the Russian laws and government acts which define how such a place would function, such as: The State Secrets Act No.5485-1/21.07.1993, Russian Federation Law No.3297-1/14.07.1992 about 3ATO, Government Act No.693/11.06.1996 about the "3ATO", or the acts which regulate a particular city, such as the Ozersk Town Council Act No.205/09.12.1998 about the entry/exit procedures of the Ozersk 3ATO.
You may also find some articles such as this one from the Carnegie Corporation of NY, "Opening the Closed Cities of the Soviet Union":
The secrecy was there to protect strategically important assets such as the military installations, missiles, the space program, naval fleet etc. The "secret towns" were regulated by the Interior Ministry, but also by the Ministry of Atomic Energy and the Federal Security Service. The amount of people needed was too large for a regular military base, so they were indeed "towns", just fenced off. Some would even have a college or a university providing specialized education matching the town's industry. There would be no road signs pointing to the town, no markings on maps and even no proper name. The name of the town would often be that of a neighbouring town with a number added to differentiate between the two for mail purposes etc. The population or any other statistic of the secret towns would not appear in public information sources such as the census or news outlets. Access and entry would be highly restricted. Regular bus or train routes would not take you to a "secret town" and even if you reached the place somehow you may need special permission to enter. Not only was photography restricted, but even your mere presence in the vicinity could be a crime. There was, of course, proper signage, marking a "forbidden zone".
Only Russian citizens could live permanently in such towns. Foreigners were usually not allowed in on private matters such as sightseeing or visiting friends. The Russians had to ask permission from the Security services to visit. Hiring people in town had to be approved by the head of town administration. Getting their relatives in would also require a special permission and could be limited. Leaving the town to live elsewhere also required permission and special instructions about secrecy procedures. Higher wages, large accommodation and good facilities, fabulous shopping and security unavailable elsewhere were offered to those willing to work in 3ATO, so despite the limitations, life there was considered attractive overall. Breaching the secrecy was a crime and even visitors to the town had to be acquainted with the strict rules. Getting unauthorized people in, even if they were children, was a crime. Arguing with the sentry at the entry point was a no-no: offending the sentry was a crime, and so was refusing to comply with the sentry's orders.
Today some of those towns are open but others remain quite restricted and even quite secret. I'm not aware of another country with secrecy on such scale, although some countries such as China may still have places with restricted access, especially limiting access of foreign citizens and requiring special permissions.
The other answer goes into some good detail about Russian secret towns, so I'll answer your question about other countries by discussing the US. I can't speak for any others.
Sources: I grew up in what used to be a "secret city" in the US (Oak Ridge, TN). I'm going to avoid anecdotal evidence as much as possible (I had family who worked on the secret project there, so it's hard to avoid completely) and try to keep it to academic information and information from books, specifically "City Behind a Fence: Oak Ridge, Tennessee 1942 - 1946" and "The Girls of the Atomic City: The Untold Story of the Women Who Helped Win WWII," both books I own and would highly recommend if you're interested in the topic.
The United States had at least two that were part of the Manhattan Project: Oak Ridge, Tennessee and Los Alamos, New Mexico. Oak Ridge enriched uranium for the Little Boy bomb (separating U-235 and U-238 in mixed "raw" uranium), and Los Alamos assembled and handled testing of the bombs. Both towns today continue the tradition by being home to Oak Ridge National Laboratory and the Y-12 National Security Complex (in Oak Ridge), and Los Alamos National Laboratory in Los Alamos. There is still a lot of sensitive, classified activity going on in both towns, but neither is actually "secret" any more, although Oak Ridge was not on maps for a while. I'm going to focus more on Oak Ridge, since I know about it. Los Alamos' story seems similar. Unfortunately, the article you linked is about Oak Ridge, so I’m not sure how much new information will be provided here.
Oak Ridge's mere existence was not a secret. The Army displaced about one thousand farming families and turned a rural agricultural area into a city of 75,000 people in a short amount of time. With one of the larger cities in TN (Knoxville) right next door, people knew something was up. The town was given the unassuming name of "Clinton Engineer Works" and residents were given cover stories, sometimes that they were building housing for officers to retire to after the war. Visiting scientists staying in the guest houses would often come and go under assumed names. If you were to fly over the city, you would immediately know something was going on: the uranium enrichment facilities (gaseous diffusion building at K-25, where my grandfather worked, and the calutrons at Y-12) were HUGE, and certainly don't look like anything related to building houses.
What was important was that what was going on was kept secret. This was done by compartmentalizing information: people working on the Manhattan Project at any level other than the highest didn't know what the end goal was until after the bombs fell. Access to the town was controlled. The hills on either side made walking into Oak Ridge difficult, and there were guard towers at all the road entrances (that are still there, preserved for historical reasons but no longer in use). Residents had badges that allowed them into the city, and everyone was checked. You couldn't have guests from out of town (easily, at least), although you were free to come and go if you were allowed to be there (I'm sure this was logged somehow). It was during the war and there was a lot of rationing, especially of gasoline, so I'm not sure how often "quick trips" out of town were made, but you could certainly go visit family out of town, as long as you didn't talk about what you did.
Basically what it came down to was to vet the people who lived there, and keep people who didn't live there out. Build it out of the way, give it an unassuming name, and make sure people have cover stories. It also helps to ensure all routine services are present in town so people are less likely to make frequent trips out. It was hard to keep a project of that scale completely secret, but you could obfuscate what was going on there - and the Army did so pretty successfully.
We still have my grandmother's WWII city ID somewhere. I wish I had a digital copy to post.
Edit: I did forget one thing that goes with what I said in the last paragraph. Nobody who worked in Oak Ridge during the war lived outside the city, unless they were scientists who went between Project sites. Housing was provided by the Army, even to civilian workers, so you didn't really need to look elsewhere. Keeping everyone inside the fence ensured a lot of social connections existed inside the fence, minimizing the change of something slipping. If you drive around the wartime section of town, you'll realize that all the houses look alike. To get it built quickly, there are only a handful of floor plans that were used, identified by letter ("A" being the smallest, "D" being the biggest single-family residence, and anything above "D" being a multifamily unit). There were also some temporary houses called either hutments or flat tops depending on how crappy they were - all the hutments are gone now, but some of the tiny flat tops are still around.