Late Game Event: Bad Walls

[Greenbear]

Since many find mountain bases and fortresses hiding behind killboxes too boring during the late game, here's an idea for an event:

"One of your colonists noticed that some of your walls and roofs have developed cracks. They were either badly designed or put under too much strain. The walls will collaps in 24 hours and bury everything beneath. Clear the affected area or replace the cracked parts in time to avoid damage."

The wall and roof tiles affected will display a message similar to blights on plants, maybe the message can even show the affected radius of a roof collapse. Once the time runs out and even one affected tile is left a roof collapse will be triggered in a small radius around the tile.

This event would only be triggered two years after the game started or the wall was built.