I am in Wellington and had a nice house.
I have been logging in whenever I got a break at work, always using a Repair Kit to check my walls, making sure all at 100%. Sometimes 2 hours would pass between checks.
Then, randomly it seems, when I checked after 1.5 hours 3 walls had disappeared! Not cracked, completely gone after only 90 mins. Stone walls built on stone tiles.
Any clue why?
Be sure to use a Scholar’s Lens to see who built your walls. One of the oldest tricks in the game is putting your walls in someone else’s base and then using them to break in when the person isn’t online. That is almost certainly what just happened to you.
Among what DF3CT said, make sure all perimeter can form a square/rectangle shape (eg 2×2 or 3×3, not 3×1), or a wall that isn’t connecting to anything… Any wall that doesn’t conform to that pattern will quickly decay.