The previous owners of my home took out a tiled shower and a second doorway to put in a bathtub. The original floor tile remains but there is a section that is the old shower pan. It's level with the floor tile. And it was covered with peel 'n' stick tile (which doesn't stick to old concrete).

Can I tile over the old tile as well as the old mud thing or vinyl over it all or does it need to be removed? The tile (and mud) is very flat and stable. This would only need to last a few years or however long it takes us to get around to actually fixing the bathroom. Doesn't need to be super classy, but it would be nice to be more presentable than this: