How can a person shoot at the White House?

The Secret Service has arrested a person who fired two shots at the White House.
Even though the President and First Family were out of town, the media hasn't quite focused on how is it possible for a person to fire at the President's home in the first place?  What about a secure perimeter?  What about some type of shield?  Hopefully there's something more that we don't know about when the President and First Family are actually there.  But this is scary.

I remember back in the late 1980's when I was at Seton Hall University and Vice President Quayle visited the campus.  I wasn't frisked, my belongings weren't searched.  Yet, the VP walked within 10 feet of me.  I hope the same thing isn't happening with the President today.

I hope all this just shows my ignorance rather than lapses in security.  Or maybe in reality a President really can't be protected from a determined person.  But I think we should do more to make sure it's not possible to harm our valuable public servants.  I would think at the White House there could be near-see through protection that could stop bullets.  Also, the area within an assault weapon's range should be secured for firearms.