Making "rooms" inside "buildings"

I've been trying out the SRB2 Doom Editor since I have an itch for creating a level, and there's one idea I have but I'm not too sure on how to do it (then again, I started doing this literally an hour ago). I've been messing around and trying to make it work but I can't seem to, so I'm wondering if it's even possible.

My level concept is that it takes place high in the skyscrapers/buildings of a city, so it's basically a giant death pit with platforms in the air. One of the things I want to do is have the main route be on the roofs of the skyscrapers, but skilled players can jump down and go IN the skyscrapers for faster routes/items (basically, I'm inverting the "go higher to go faster" idea that usually holds true for Sonic levels--here you want to go as LOW as possible). I tried making this work, by making a very tall sector representing a skyscraper, and then making a smaller sector inside of it that borders its outside edge (with the ceiling and floor raised to be between those of the bigger sector) , but I don't how to make it a "room", to speak, so that you can jump inside it from the outside. I can't just delete one of the linedefs because that makes everything all screwy, so how exactly is this done? I can make it so that you can go IN, but you can't actually go back OUT (though it looks glitchy as hell). I know it is possible, the official levels have plenty of rooms inside of "buildings".

Hell am I even doing this the right way? I don't know, I'm just trying to mess around and see what works.

EDIT: Okay, I SORTA figured out how to make holes in a wall, but it seems none of the walls are solid, and when you enter and look back, you just see another wall.

Sounds like you'll need some of these: http://wiki.srb2.org/wiki/Floor_over_floor

Oh that seems like something I need. I'll try those out.

What you need to do is make the room inside the building first, built normally with standard linedefs and sectors. That should result in a normal-looking stage with no ceiling. Then, place a floating platform on top of the whole thing to create the ceiling of the room and the rooftop above.

Think of map-making in SRB2 like building something with Lego. If you're building a house, you create the interior of the house first then put a roof on top at the end.

Okay this is not as relevant but I don't want to clutter the board up with new threads.

I can't seem to get two sectors to stick together in SRB2 Doom Editor. When I do either of the two methods the wiki mentions:

Now drag it even further to the left until its leftmost linedef is right on top over the rightmost linedef of the other sector. If you let go of the mouse now, the linedefs will merge into one. This happens because Stitch Vertices is checked, which makes the overlapping vertices stitch together. Another method is to go into Vertices Mode (you will have to undo the last few actions until the sectors are separate again) and drag the two leftmost vertices of the right sector to overlap with the two most rightmost vertices of the left sector. In this case however, the sector whose vertices you moved will change its shape, which must be manually adjusted afterwards....

....Zoom in on what should become the shared border of the two sectors. Then, in Lines Mode, right-click near the vertex on one end of the border without highlighting any of the already existing linedefs. If one of them is automatically highlighted, you must zoom even closer. You should now be able to draw a new linedef from the selected vertex. Now draw the geometry of the new sector until you end up at the vertex at the other end of the border. Place a new vertex on it and right-click. The new sector will be created and the linedefs that belong to the original sector will have their references changed so that both sectors are closed.

Click to expand...

All I get is a new sector that overlaps with the second sector. Oddly enough, in both cases, when I move it around with sector mode, the new sectors act like they actually are connected, altering the shape of the original sector, but they still get treated like separate sectors...?

EDIT: Never mind figured it out.