Q: Can you lay new tiles (saltillo) on top of old ceramic tiles?
A: Yes, but only if the existing tile surface is in good shape without cracks or loose tile. The tiles need to be cleaned of any grease, oil or other bond breakers. Use a belt sander or angle grinder to scuff the glazed surface of the exiting tiles for better grip. Install with modified thinset mortar, and things will stick nicely. Also, make sure your subfloor can carry the weight.