Overtext is really cool. The best 100 GG I've ever spent.
Well. I'm actually bored with overtext. Why did I give all that GG away?
The rules say
Each falcon must be placed (face up) on the intersection of four building tiles of the player’s own Alhambra ...
Falcons can be placed on starting fountains, ...
But Portals are no building tiles:
Portals cannot be rebuilt or kept in reserve. A building tile touching a portal may be rebuilt only if the portal still touches at least one building tile of its color after rebuilding is complete.
Portals are treated in a completely different way from building tiles.
That's why I wouldn't allow falcons on a portal.