Can tiles be laid on old tiles?

In principle, certainly!

We recommend the following procedure:

the tile covering must first of all be tapped, loose tiles must be removed and the resultant gaps levelled out with mortar. After cleaning the covering of any soiling, a special primer coat is applied. The tile adhesive is then applied with a toothed trowel and the tiles subsequently attached. The instructions issued by Deutsche Bauchemie must be observed.