Fix;

js/app.js

@@ -147,7 +147,7 @@ let game = new Vue({
             let savedBuildings = JSON.parse(localStorage.getItem('buildings'));
 
             if (savedBuildings.length !== this.buildings.length) {
-                localStorage.setItem('buildings', this.buildings.stringify());
+                localStorage.setItem('buildings', JSON.stringify(this.buildings));
                 alert('Buildings have been resetted due to an important update.')
             }
         },