Mostly covered in the answers so far, but to prevent cracked grout lines or ties lifting in the future take pains to ensure there is no deflection in the ply ie it is really rigid without any flexing. If floorboards were first secured, a screed of flexible adhesive applied, and then min 12mm WBP plywood (Water and Boil Proof) used and screwed down at 150mm centres, with both sides sealed with bonding solution, then you should be good to go. If not, remove plywood and go with 6mm backerboard as recommended by aussietiling earlier. If you don't do much tiling remember to read the instructions and measure accurately when mixing the adhesive to ensure good performance and avoid problems with poor adhesion or crumbling