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Plant pot shelves up on the wall.
Random windows in interior walls.
I'd quite like to do something like this except with a water feature.
That stone archway set in plastered walls looks great.
Table that pops out of the floor? YES.
Changes in flooring colour.
Plant low, thorny bushes beneath your windows. Bougainvillea, barberries, roses or any prickly specimen ought to deter break-ins. Yuccas and other large succulents are a lower-maintenance option.
Shelving up around the walls.
Tall, thin windows in atrium.
Groin vault ceiling.
Shades for south-facing windows (in the northern hemisphere), for example, should be horizontal to address the high sun, but a western facade (such as the one shown here) should be vertical to deal with the low sun as it moves on its horizontal course across the sky.
Dark wood aesthetic.
Completely inaccessible nook for displaying art.
Arch with little staircase. Floating landing.
Big domed window at side of barrel roof.
Interior or exterior stone arches.
Corner window in random outlying shape. Cute.