The Army's doctrine for urban combat (MOUT--Military Operations in Urban Terrain) call for attempting to assault multi-story buildings from the
top-down in recognition of the inherent difficulty in assaulting upwards against a defender.
You said that the stairs come up into the living room so I assume that the door is actually at the foot of the stairs on the ground floor. This is ideal. You should get some notice that your door has been breached but it is now uphill for the intruder. Try this: Looking straight ahead see how far you can rotate your head and what field of vision you have. Then look upwards and try the same range of vision test. It is severely limited.
I recommend keeping a pot of boiling oil simmering on the stove.