Level Design Explanation

Hi guys, i want to know one think about level design and i dont know if im in right place but wt i would like to now is when we made a game type Diablo or Tourchlight how we made the house wall and some dungeon walls disapear for a certain time when we are near they to show wts going on in other side? Are that mayde by scripting or object properties?

Sry if this is not the right place... _And if so move it for the right one.

Appreciate
Grtz