Removing tile from older-style mortar walls can be very challenging. It depends on a lot of variables in place at the time the tile was set, but it comes down to how well the tile is bonded to the mortar. Sometimes you can get a flat chisel behind the tiles and pop them off one at a time almost whole. Other times the bond is such that the tile, thinset and mortar are almost like a single unit fused together. There is no magic formula, and the job takes a good deal of time and effort.
Put the chisel along the top edge of the tile trim and hammer down. These pieces should break off fairly easy. Continue chipping until you have removed the trim pieces and have exposed the top edge of the flat tile.
Place the blade of the chisel behind the edge of the tile. Use a wide, sharp chisel. Hold the chisel in place and strike sharply down on it with the hammer. Depending on the type of installation, you may be able to pop the tiles off the mortar without too much effort. Continue down the wall, chiseling off the tiles from behind wherever possible.
Strike the face of the remaining tiles with the hammer to break them up. You aren’t trying to bust through the concrete; you are trying to break the tiles into smaller pieces to make them easier to chisel off. Once you have broken up the tile, repeat the process with the chisel behind the edges to break off the remaining pieces of tile.
Chip away any remaining thinset that is adhered to the concrete. If the entire wall is still covered with a layer of thinset adhered to the concrete, just chisel off any high places, leaving a smooth layer that you can tile back over.