The Arizona Ranger

So to clarify, just placing a bunch of props and a fading door in a tunnel between districts, such as the tunnel between district 1 and 2 would not constitute a mega base,

Mega-basing appears as a topic covered in the rules but it seems pretty ill-defined. From what the rules say I get the impression that it is a base that spans multiple ownable buildings. However, I've seen players build in the streets and tunnels claiming that is a mega base. Is this true? Because it would appear to me that megabases, like normal bases, require you to own certain doors.