My apartment is in a very large block with several separate entrances in different streets, so no-one can really tell where callers are headed from the outside. In addition, the entrance to my bit is in a side street and opposite commercial premises; one of my windows overlooks it but none of the others do, and I find that my neighbours are rarely encountered in the corridors (plus as some of the flats are holiday lets, subsequently they are not only empty a lot, but the other residents are fairly accustomed to different/unfamiliar people coming and going).
I would say that your main area of concern is your neighbours - these are the people who will make your life a misery if you are found out. When you say there are five flats in the courtyard, do any of them have windows which overlook your door? Are the people who live there elderly/looking after young families and likely to be in a lot during the day? Is the gate buzzer very loud (and for that matter, does the room you plan to work from adjoin any of the other flats? Are the walls thick?)
Don't worry too much about the outside appearance of the building - you can always make a bit of a joke about it on the phone to new visitors, and provided the area isn't a really bad one it shouldn't matter too much. My last place was down a back alley in a far from salubrious part of town, and I managed OK! It is also true that the current media portrayal of us can actually work a little in your favour - as Sophia said, provided you're not seeing 10 blokes a day and don't answer the door in inch-thick makeup and thigh boots, you're unlikely to be sussed!