Defend your city
Build military facilities to be able to defend against enemy bombings. Increase the range of your defences so you can expand without fear of being destroyed by bombardment.
The attacks will get stronger and stronger and you will need better defences if you don't want to lose parts of your city.
New biome: Mountains
Food is hard to come by in a frozen world. Give it a try! Although the snow will bring new tourism options, such as ski slopes.