I want to plant a shade tree on the west side of my backyard but that is where the ground has about a 20-30 degree slope away from a stone fence property line.

My first thought was to build a 3 sided planter box of some kind, (the 4th side would be my stone fence), and fill it with dirt to have a level spot for the tree. The planter box would be about 10' square with a height of maybe 2' on the front side.

Is this a bad idea or should I just plant the tree in the inclined ground as it is? I've seen landscape pics of yards here in Vegas where they have a tree in a 2' high brick planter and the rest of the yard is all concrete, basically an island of dirt in the middle of the yard.

Also, how far from the stone fence can I plant a tree? I'm thinking around 5 feet.