There's a ringworld down in the middle. Size 300 systems means that they're actually pretty small, about 1 grid square?

If you mean this arrangement of rings, that's via abusing Fling Beacons and Level 5 FTL planets to rearrange everything to my liking.

The arrangements are: Level 1s on outermost ring, Level 0s, shipyard bases (FTLed into position), Level 2s, Level 3s then Level 5s and the special buildings (like Ringworld)

the ringworld thing? or how do you do that?