.we never even get to see where that funky louvered door leads to.
If you look at that door, it's not really even a louvered door. (I always thought it was until I got a closer look one time.) There are slats with a small open space between them, and they're not at an angle at all, nor do they move.
I imagine that the door leads to a servants hallway, the kitchen, back stairs, etc. That makes more sense than having the basement door within viewing range of the front door, as it was by the time Nicholas showed up. Like lots of places on the show, they got closer and closer to each other as time moved on!