This detail makes all the difference:
... Secret rebel hideouts ...
While there are many valid answers for securing something up high, a secret rebel hideout has extra considerations that matter. For one, secrecy is just as important as security. An airship or mountain top fortress might be hard to access, but they stand out against their backgrounds such that surveillance is bound to detect them. As for the hideout part, radio towers will be way too small.
This makes skyscrapers the ideal location
Because a skyscraper is already host to thousands of visitors a day, and has a massive logistics need, you can drive trucks full of supplies and rebels in and out of the place all day long and no one will see anything unusual happening there. This means no amount of aerial reconnaissance will reveal your location.
While one other answer suggests using unofficial space like mechanical floors, such places will be prone to inspection by outside agencies and will have no legal stop-gap in place to keep law-enforcement or fire inspectors out. Using the building's official space would be even better.
Because it is common for skyscrapers to host a variety of data sensitive civilian firms (Banking institutions, IT companies, medical records firms, law offices, etc), high security in parts of the building is 100% to be expected. All you need to do is buy up a bunch of office space and establish a small front office that looks legitimate. As long as your front should legitimately be following an escalated security standard like PCI, CMMC, HIPPA, etc. then no one will really question why you have such strict visitor policies, plus it gives you rights to limit certain kinds of access to law enforcement.
For example, let's say a cop shows up and he has a warrant to search your "medical archives company", because of HIPPA, his warrant will generally be very limited in scope. In most cases, your office staff can prevent the officer from doing a physical search under the grounds that a cop could not reasonably search for the thing of interest without going through records which are protected under HIPPA and are not part of his investigation; so, it would be common practice for your archive staff to fetch things of interest (like specific medical records) for the cops rather than granting physical access.
Or another example might be if a fire inspector drops by unannounced. If you were hiding in the mechanical floors, you would be discovered. However, if as a high security firm, you can tell him you need time to secure your sensitive information and schedule him to come back later after you've had time to hide all of your illegal, I mean "sensitive" materials.
As a last line of defense, a skyscraper has the added advantage of being positioned right above the heads of thousands of innocent civilians. This means that if your location is discovered, they can't just call in an airstrike without incurring massive political fallout. If they want to assault your position, they have to go in by foot without the advantages of air or artillery support. This gives your rebels a fighting chance to hold the position long enough to destroy records, await reinforcements, or if they are fanatical enough, to detonate any self-destruct devices there might be.