Ceramic tiles are bonded to the wall using one of a number of different adhesives. These tough adhesives dry to a rock-hard bond between the tile and the wall it is installed on. Some of these adhesives are water-soluble and fairly easy to remove, while others require chemicals to soften them enough to get them off the wall. If you have managed to remove the wall tiles without damaging the wall behind them, an adhesive remover is what is needed to safely to get the glue off as well.