RE: 1) Ruin reconstruction
For castles and towers, I'm a little skeptical. Would a second medieval
engineer trust the construction techniques of the guy who built before? Or
perhaps a 1% chance that the new construction would be built on a faulty
foundation and collapse when first attacked.
RE: 2) Stones from Ruins
RE: 3) Lairs to towers.
I don't know. Seems to me that a lot of monsters would build lairs in places
which were hidden, but with easy access to prey. Towers tend to be built in
commanding locations so that they can dominate a region.
RE: 4) Adding to castles
I think this is a great idea and would add a lot to the game. It would give
advanced empires something to do besides beating up on their neighbors. And
it would allow a strategic location to really be nailed down; there's lots of
precedents for that in Earth history. Hmm, then maybe we would need some
treachery rules for getting gates left open.