We consider a brane world residing in the interior region inside the horizo
n of the D3-brane. The horizon size can be interpreted as the compactificat
ion size. The macroscopically large size of extra dimensions then can be de
rived from the underlying string theory that has only one physical scale, i
.e. the string scale. Then, the hierarchy between the string scale and the
Planck scale is provided by Ramon-Ramon charge of the D3-brane. This pictur
e also offers a new perspective on various issues associated with the brane
world scenarios including the cosmological constant.