The Tutorial level is always marked with a silver skull. Even when you finish it with 3 stars.

When i then finished the game and had every city marked with a golden skull, the city auto turned to gold.
But after a crash in one level. The tutorial city was back to silver.

Ohh and... if you bring out to many units or conquer to fast. The game with crash to desktop.

OHHHHH AND ... The dragon has no in-game symbol displayed when you have upgrade for it from buildings.

OHHHHHHH ANNNND .... The dragon will soft-lock the level when it dies to a barricade.