Tiling on top of tiles

[Conshine]

I am re-tiling part of the bathroom.

One of the walls is currently tiled - exterior wall.
The other is not - plasterboard.

I believe it will prove very difficult to get the old tiles off, as the wall is very solid and the tiles seem to be stuck on very securely.

I am considering getting a hammer & chisel to get off what I can and make as rough a surface as I can to ensure that the new tiles have something to form a good grip on.

Does anybody forsee any problem with this?

I assume tiling directly on to of old tiles (without the above effort) is not a good idea?

 

[Caveat]

I assume tiling directly on to of old tiles (without the above effort) is not a good idea?

I have seen it, but it doesn't strike me as a good idea.