The details on respawning AI civs:
AI civs can only be respawned once.
For an AI civ to respawn, there must be an unclaimed, unoccupied tile on the same continent as the one where it was eliminated (by killing it's last settler or last city) and no other civs can have territory or units within a 5-tile radius of this tile. The tile must also allow cities to be built upon it (i.e., no mountains/water using default rules).
The respawned civ loses the following:
All embassies/spies
All units
- All colonies
The respawned civ gets the following:
1 city
3 defensive units
1 offensive unit
1 settler/start unit 1
1 worker/start unit 2
All bonus starting units based on difficulty level
Their gold is increased by starting_gold X 10 (i.e., 100 gold using default rules).
Everything else (culture, at war, techs, etc.) remains unchanged.