The set is beautiful. The specs on the bed stated no box spring required  minimum mattress height 6 inches. 

I ended up buying a 9 inch mattress to add a little height, being that the headboard is tall. I put everything together  placed the 9 inch mattress on top of the slats. It was so low  looked completely off. 

I bought a 5 inch box spring  the 9 inch mattress and it looks so much better.