When you created your terrain, you stamped rectangles onto it in case you wanted to remove them later. Have a go at this now:
Select an area where you would like your buildings to be lower down—perhaps you've placed buildings there and the area is just too hilly.
Go to View and select View Hidden Geometry.
Now, edit the terrain and use the Erase tool on the rectangle as shown next:
You can remove all of it, or as much as you want.
When you're done, drop a blue line down one corner as shown in the next screenshot, and then join it up to all other corners, making sure you stick to the Red, Green, and Blue axes only.
Draw vertical lines up at each corner. You should finish up with a rectangular hole.
You can now Push/Pull the base up as high as it will go and paste a texture in there.
Place the buildings in the hole.
First of all, you used your modeling skills to delete the area that you stamped out in Chapter 5, Game Levels in SketchUp, then...