Building an unbreakable spawn above the height limit

When you start a server I believe you can increase the height limit of building. Before I attempt to figure this out I wanted to know, if I increase the limit to 512 and build my spawn above the 256 limit then set the limit back down to 256, will my spawn still be there just unbreakable? Has anyone else tried this that can guide me in the right direction? (I haven't been successful in starting my own server so I cannot test this myself)

You cannot set the build limit higher than 256, that is the hard coded limit of the game. You can, however, set the build height to a lower value (such as 200) after building your spawn above it. This will cause your spawn to remain there, but building will be impossible above the limit. I am not sure if blocks can be destroyed, however.

Also note that server height limit is not extremely precise, and may be several blocks off of where you set it.