Require only one small screw* to assemble the two half of the core. One center is empty to allow the screw to be screwed after the puzzle is assembled. This puzzle require a lot of breaking-in, you have to run each pieces against each others. I assembled mine out of the box and it didn't turn at all.
*I used a screw from an RC car that I dismantled, so I don't know exactly which type it is, all I can tell is that the core hole for the screw head is 8 mm in diameter.