Need advice on covering of old tiles

The only thing you could do is retile , you could lay new over the top to save ripping the floor up as long as there's no cracks or movement in the existing floor . If there is movement or cracks you will have to lift existing tiles and lay new ones to a properly finished floor by boarding it and using self levelling compound . Hope that helps

After unibonding the floor, using a two part floor levelling compound with latex, u can cover the tiles with a surface that's ready to work on in 24hrs.

I would recommend a damp test on the floor first as quarry tiles were often layed without a damp membrane years ago.assuming all is ok and the tiles are solid and sound then the tiles can be primed and a latex levelling compound applied then the flooring choice is yours.

Yes you can tile over existing tiled floor with no pb as long as there are no loose or cracked tiles. Remove the loose or cracked ones and pour some self leveling compound. BUT BE CAREFUL WITH THE HIGH AT THE DOOR IF YOU WANT TO TILE OVER. I am 100% sure you will have a 15 mm step at least. 10mm tile thickness and 5 mm adhesive bed. Will look awful even if you protect the edge with a fine trim. Plus you may have to cut the door underneath. As per tiling ...I will suggest a sand strong sand paper ( polisher) just to scratch the existing floor all the way around for a better grip of the next adhesive. Hoover the floor after and apply a Primer coat. Success