1. if my active race goes in decline, does the fortification stays like troll's lair ?
Fortifications stay on the board when the Fortified race goes into decline.
2. if enemy occupied my territory with a fortification, does it goes away or now belong to the enemy ?
Fortifications disappear when you abandon the region or when it is conquered.
3. if amazon, let's said, do the conquest and by the time he should remove the 4 attacking amazon in the board, he ran out of token that he had to leave one region unoccupied, how does this resolve ?
The Amazons may use their 4 extra tokens for conquest, but at the end of each (active) turn, four tokens must come off the board. If this means the Amazon player must abandon one or more regions, he must do so. The player chooses which region(s) to abandon, and does not score for the abandoned region(s).