A shop
...shop granting the wishes...
The shop is located in Tokyo, modern Japan, and it’s practically in the same environment as for other CLAMP series: Card Captor Sakura, Suki: Dakara Suki, Gouhou Drug, Kobato and Angelic Layer (not sure for the last one, but Misaki at the beginning has to go from Tokyo station to Ebisu station with Yamanote line, so I presume yes it’s modern Tokyo).
It’s called a shop because things of same values are exchanged, even
if we aren’t speaking about material stuff, but about wishes and similar.
Still this building can be seen only by those who really wish for something
and need a supernatural power to help them. 
The
same building is also protected by a barrier (jpn: kekkai) that normally is
kept up by Maru and Moro, keeping away the evil spirits and Fei Wang to find
the location. Those who don’t need the shop, can see just an empty abandoned
field between two tall buildings.
From the beginning the shop has a purpose for the final moment or how Yuuko loves to call it “for the day that has to come”. In the last volumes of Tsubasa: Reservoir Chronicle , we find out, that the shop was made after Clow stopped the time for Yuuko and they had to prepare to help the “children” to fulfill their wishes. While Clow paid Shaoran’s (clone) price, Yuuko got all that collection (look at the collection section) of artifacts to pay Sakura’s (clone) price. The two of them also created the Mokonas along with the shop, to stop Fei Wang’s wish coming true.
On the exterior
there is no signs, because simply, this is the residence of the Dimension witch,
who calls herself Yuuko Ichihara. It shows unique design in its tower and balconies,
surrounded by trees, which is located on a surprisingly expensive lot for the
middle of the town. From the yard there can be seen the nearby high-rise and
it almost seems that the shop is completely surrounded by tall buildings.
The gateposts are decorated by moons and the fence is quite high. There is a
stone walkway that leads to the front door, which is in European design. There
is also a small set of stairs placed in the way so the yard is easy reachable
from the opened hallway. The courtyard is big enough to hold all of the treasure
from the repository. On the back yard there is also a well.
While the exterior is in western design, once you enter the building you find
a completely traditional style: sliding paper doors, tatami mats, everything
that is Japanese. The visitors are greeted at the entry door facing a crescent
moon screen. There are also many tasteful decorations. The treasure repository
can be found at the end of the hall guarded by dark slider painted door, which
gives them an impression of being thick, heavy and forbidden. In the back there
is a traditional bureau, which Yuuko uses for storing her many kimonos. There
is a guest room, that has only a single small dresser. There are also other
rooms that Yuuko uses from time to time, and she likes to sit on the opened
hallway that leads to the yard.