Tiling over very old tiles?

[Skiny]

I want to tile a room, which has some very old tiles. After removing the nasty laminate flooring the previous owners had, I found tiles which must be original to the house. It was built in the 1870s and they're barely even recognizable as tiles anymore, as they seem to have merged into one solid surface. Only with lots of cleaning can you make out the lines between them. Overall it's pretty level - about a 7mm dip at it's worst. Nothing is loose at all - it's very solid and could easily be mistaken for a concrete surface.

Would it be ok to use some self-leveling compound to give an even surface and then tile straight on top of that? Would the floor need to be sealed with anything to prevent damp coming through? I guess there's just earth beneath those old tiles.

 

[vinn]

In the long run you dont want to be tiling over earth but unless you investigate you wont know whats there?
The old tiles are perhaps fixed on a mortar bed, & grouted with sand and cement.
Do you have any signs of damp in the floor or the walls?