When people think of "building their Citadel" they often think of it as a place. A Bitcoin community. Complete with concrete walls and castles maybe. However, its important to know, that it is actually a little more simple than that.
Yes, Bitcoin communities are one way to establish a Bitcoin circular economy and they are awesome. For that, you should 100% check out The Bitcoin Beach Whitepaper
However, to build your Bitcoin Citadel, it is actually a little more simple than that. Your Citadel is your personal network of Bitcoiners who will work and accept Bitcoin.
Networking is a concept in business where you have many people or contacts that you know who typically help you find jobs or business and in exchange you help them find jobs or business. In this context, this is a group of people or contacts who accept Bitcoin and/or know other people who accept Bitcoin and can get you what you're looking for. Its a process of making friends and it takes effort to do and build up.
This can later grow and build up into a full on Bitcoin community, a fully fledged circular economy, but when I talk about how people should "Build their Bitcoin Citadel" this more simple form is actually all that I'm referring to. I call it a Citadel, because it is impervious to bans, censorship, banks, everything. If it can't cut off your contact to your friends, it won't take down your Citadel.