I explained and played the game with 3 today.
A player wanted to place a city tile next to the empty Noctis City hex.
He asked if that was legal and I wasn't sure but I let him do that.
The Noctis City card wasn't played in that game, but I suppose playing was not legal then?
It is legal to place a city next to the Noctis City hex as long as it hasn't been played yet. It would still be legal to play Noctis City later as that city is an exception to the normal placement rules forbidding placement next to another city.