This is wonderful how you opened the stairs - you get a real sense of the architecture and it brings in more light. I applaud what you have accomplished. I'm going to assume that the wall above is patched with quickcrete and the whole thing is painted (try to get the wall beyond under the stair too). If you have / want a flat screen, you might mount a swing arm wall mount for a flat panel television on the vertical wall element remaining on the right - just about 3' - 3 1/2' (around 1 meter) above the floor so that it is eye level when you are sitting. The swing arm support will allow you to tilt the television away from the wall ever so slightly across the nearby corner. This can help to eliminate glare from the nearby window and make it possible for more people in the room to view it / expand your seating possibilities. I reference something like http://www.cdw.com/shop/products/Chief-Thinstall-TS318SU-Medium-Dual-Swing-Arm-Wall-Mount-18in-Extension/2555661.aspx?cm_mmc=ShoppingFeeds-_-GoogleBase-_-Video/Monitors/Cards/Projector-_-2555661_Chief%20Thinstall%20TS318SU%20Medium%20Dual%20Swing%20Arm%20Wall%20Mount%20-%2018%22%22%20Extension%20-%20_CIF-TS318SU You can even mount a television tlhat is wider than the wall behind as it can sit flat on the wall and extend beyond the wall to the left when at rest like another plane in your architectural composition. Since the upper stair is just behind, this should be fairly safe for the electronic. Determine precisely where the television will be at rest before you hang the mount itself. Then, look for a low height chest to sit below the television. Ideally a chest with one open shelf near the top to hold your component receivers. You may want to try to run electrical to the remaining vertical wall on the right (the support for the stair landing) to get power into the wall itself, while everything is open. Depending upon where the nearest source is, with concrete walls, you could mount the power junction box low on the wall under the stair and drill out small diameter cord passages through the wall after placement of your components. Or use the baseboard area to run needed cables and power to the area behind the chest, then conceal it with baseboard trim. The more you can hide the power and connection cords, the sleeker the look, to match your gorgeous architectural stair element. If you have a larger deeper television, put it on a sideboard chest under the upper stair where you have the depth. In the rear area, take care with putting cables together in a cord cover - ikea has a good one that is inexpensive. http://www.ikea.com/us/en/catalog/products/20097954/ so it looks "neat" from behind when you come down the stair.

How do you decorate your place, what kind of style, colors, and feelings? That will greatly influence the type of door you might want. Also, think of your budget, and whether you want to install it yourself or hire a professional (you don't need to tell us, just keep that in mind). Then look around Houzz, in the products page, they have lots of interior sliding doors and hinged ones, from elegant frosted glass in a dark wood frame, to rustic reclaimed wood planks. You may not want to purchase that exact model, but it might give you a better idea of what would fulfill your airflow needs without sacrificing style.

Scope of work would benefit from consulting a security company.

The amazing thing is that you have the luxury of approaching the architect about these issues. Laminated glass should address the security along with any alarm or security system you plan on installing, though the laminated glass may have size limitations. Regarding the waterproofing you should contact the architect, contactor and waterproofing subcontractor, or see about hiring an independent waterproofing specialist who can detail and warranty the condition. As a design consideration, make the lower area a garden or gallery or sculpture area and upper should be all glass. Maybe a garden roof there too if accessible or visible. In any case looks like an amazing place!!