Flooring Tile - Tiling over a mud job

I am planning on retiling my bathroom walls. I have taken off some of the old tiles and noticed that it is a mud job. The concrete appears to be in good shape. The old concrete looks like square blocks with the corners cut off. Can I retile directly over the concrete or would it be smarter to lay cement backerboard first? If I can retile directly over, how should I prepare the walls? Do I need to fill in the small gaps in the mortar bed? Please help.

If the wall is pretty good shape and reasonably flat then fill the holes and start tiling.Make sure the concrete is still sound and not crumbling or disintegrating.Nothing special to do as far as i can tell.

I'm curious though ,did all the tile come off that easily that it left all that in good shape? :cool:

I'm not done yet, but I have finished taking down one wall and only one piece of concrete is loose. The rest is very solid. The tiles came down pretty easily. The most work I had to do was tap on a 4in taping knife to pop some of the tiles off, but it was easy.

What do you recommend I use to fill the holes?

Thanks for your quick response.