what tileset? the cathedra/crypt?
For the Tombs, they should be a kind of valley of the kings type of thing. Corridors going deep, rooms, chambers, gaps in the wall leading into caverns opened by excavations. Flooded areas etc.
For tilesets, any of the Dungeon statics will be fine. Ayleid ruins are quite white and almost luminous and can be used but without any of the very specific Ayleid emblems as they are just too Oblivionesque. However in some cases these can be used so long as they are not overt and put together in such a way as to just look like another Ayleid ruin in another world.
Ruined fort, caves and so forth are perfect as they are quite slimy, green, damp looking etc. Ive not tried the cathedral crypt etc stuff but yes they could be used in some locations, though they might look just a bit too much like a cellar rather than an old extensive tomb along the lines of mountain excavations etc.
Elves in Mesogea? Not really, but something kind of similar.
The Sheahlin are an ancient people, one of the principal earliest men, bred initially as slaves to the Vandur (and providing food just as cattle do humans today). The Sheahlin survived the Mesogean ice age in the far north, a hardy people who now live on numerous islands that are constantly besieged by Vanduric forces. The Sheahlin are excellent sailors, longer lived than most men (300-400 years) and have a knowledge of the earth and skills that others might consider almost sorcery. They are accomplished soldiers, regimented, organised, disciplined and intelligent. The closest warriors of history to them would be the Spartans, though their politics and morals would be very different.
The Sheahlin were once more widespread than they are today, and it would be no surprise for them to have abandoned cities across Mesogea, many of them underground.