How to discourage trespassing neighbors?
If the size of your courtyard permits (it sounds like it might not, if their door really does open onto your space), maybe you could get some privacy screens, potted plants or even lattice+climbing vines to delineate the space and give you a more of the privacy you seek without actually building a fence. It sounds as if the layout of the property is your enemy here, not really the neighbors. Being civil and working around the constraints must be the best option short of moving to a more suitable location.
Adding furniture may result in the neighbors thinking “Cool! New furniture!” and making themselves even more at home. I would ask the landlord to “remind” them of the set-up. You are paying for the courtyard, they are not. Even if the landlord tells them, you may find yourself having to deal directly with them. Like you, I don’t like to upset or anger anyone more than necessary. But you can diplomatically address it in a manner that is not “Get off my lawn!!!!!” style. Tell them that part of the reason you rented the place, and the reason you are paying what you are (which is none of their business), is for exclusive use of the courtyard and it’s nothing personal against them. Hopefully they are not jerks. Keep in mind, we always want to tell people upsetting things, without them getting upset. It doesn’t usually work that way. Let them be upset, as long as they don’t take it out on you.
We were told verbally before leasing the apartment that the courtyard would be exclusively ours, and not shared with the other units. This was significant to us, as there is very little privacy between the courtyard and our living space. This is not, however, explicit in our lease (I know — we screwed up here). You also screwed up when they showed you the place, you noticed a door and a staircase leading into the courtyard, and thought it could somehow be exclusively yours. In NYC we don’t have so many courtyards, but we do have roofs, which are generally nice to go hang out on. Some are set up as common space for everyone in the building. Others are are off limits to tenants, which practically also means that they are common space for everyone in the building, and people will only yell at you for being there if your noise bothers them. The average landlord’s efforts to keep people off are generally rejected – if they put a “Fire Exit Only – Alarm Will Sound If Door Is Opened” someone
Your neighbors have already made it clear that they will ignore your wishes with respect to use of the courtyard. This is not an issue big enough to risk having angry neighbors, and it’s probably not worth being angry neighbors about, either. I don’t believe there’s any change that you can make to the current situation to make it better for you than it already is; better just to let it go. Ah, I see you already made contact with them a second time, and because the wife was more civil, you believe the problem is solved. Good luck with your pissed-off neighbors; you’re going to need it.