The model comes with couple of options: You can have it planted on a base with a bearing so it can be placed on a table or shelf and rotated effortlessly, or you can print the tree crown with a loop for hanging the model high up (as sky islands are supposed to be).
The island part is hollow and the city part is hollowed out enough to let light out while still printable completely support free. For assembly you can print the pegs to have the city part work as a lid, if you want to be able to open it up at any point. How well these fit will be up to your printer, might have to scale up or down as I can't say how precise my printers are in the end. You can also opt to just glue it all together as more permanent solution.