The game is balanced around the defense actions being used as inexpensive "tank one or two hits in an emergency" options, so yeah, the barriers aren't really meant to be stacked repeatedly or kept up for more than a couple turns
And about the "sudden closing the game"? Have you found anything in regards to that?
No idea! If there was no error message, and you don't know what caused it or how to recreate it, then there's nothing I can do about it, unfortunately
