Question
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Followup To
Question -
I've just purchased a cool old loft with a 23x14' terrace. It will
overlook the top of a garage to the front and a railroad track some
distance away to the right. I want to walk through my front door
and see outside to my peaceful terrace. I want height of trees or
bushes, so it will look fairly enclosed.  It faces north.
Answer -
Hi there!

In order to give you a better answer, I would need the following additional information:

How much weight would the strucuture support?
Would it withstand some rocks or gravel?
How tall would you want your trees/bushes to be?

The structure will actually be sitting atop a flat roof, therefore, it will withstand rocks or gravel. I would like trees and bushes to be no higher than about 5' tall so it feels enclosed but with a sense of space beyond.  

Answer
Try to find some stones that are covered mith moss and liquen. One of them should have a triangular mountain top shape. It should be about 3' high. This would be considered the main stone. On the left side of the terrace mound up some dirt, as to form a little hill with the triangular stone on top. (Use soil that is rich in clay so that it will be firmly set)
After that, place another stone to the left of the first one. Place another stone to the right of the main stone.
The main stone should be behind both of them.
The shape of the secondary stones should either be more flat, or a bit round.
The prportion of their sizes should be 3-2-1
If the tallest one has a 3'heigth the second one should have 2'and the third 1'.
A nice little pine tree, (since you're facing North, a japanese white pine would do better with less sunlight) could be planted behind the rocks
You could make a second mound to the left of the first, and another to the right. They shoud be slightly to the front of the main mound. Their proportion should also be 3-2-1.
On top of them you could place some odd numbered stones set in a triangular shape also. Behind these mounds you could also plant some evergreens such as Hinoki Cypress or Cryptomeria. Always maintaining a 3-2-1 proportion.
They should be able to frame the space beyond.
Spread some pebbles around the mounds and use some round stones to retain them.
Do it in an uneven pattern, so that the mounds would appear to be islands on the sea. Set some stepping stones from the door to the islands.
The setpping stones should not be set in a straight line. but in a "meandering"pattern.
Cover the dirt with moss or with some ground cover that would not be so tall as to compete with the other plants.

Good luck!

(If you could send me a picture of the terrace, I could do a little drawing for you.)
Send picture to Carlo@ organiclandscaping.com