For the most part, buildings act as impassable terrain (the rules for which were given earlier in this chapter). That said, we do allow units to enter buildings and 'garrison' them. This might seem a bit strange, but it's nice and clear in terms of rules. Basically, we don't want units moving through a building as if it were not there, and nor do we want them 'sitting' on top of it. If it's a building and you're a unit, you're either garrisoning, or not in contact with it.