I think so. I assumed that she had upstairs neighbors and could just go up to the "ceiling" of the balcony, but even if that's not the case, some sort of "roof" would be needed. Even a fish net roof.

it's not really a balcony--it's more of a porch area. I call it a balcony because it's on the side of a cliff, but it's on the ground floor.

Thanks for all your suggestions. I think I really need a live handyperson to eyeball it.

If this is a dwelling you do not own and/or do not feel comfortable making permanant attatchments to...

I think your two best solutions would be:
Set up a kennel type 'cage', like the ones I posted. You will have to take the cats to/from it and have food/water and litter available to them. You can place shelves and perches in there for them in addition to wiring small-ish tree-limbs for them to climb and scratch on.

Set up a kennel type 'cage', using one of the sliding/screen door inserts with a cat flap, attaching your tunnel firmly to it, which leads directly to the cat kennel.

Both of these solutions are not permanant, but can be made secure to insure against any escapes. I personally think the cat-tunnel would be the weakest link.

this is pretty much what I had in mind, but I'm not sure I could do this without help.

I've offered and been quietly rebuffed.

I think I could fit you in in between replacing the torn shingles and building a chimney.

I'll be home this weekend...

IMO, if you don't have any carpentry skills/tools/materials, anything you want to do that will be SECURE for your kitties, is probably going to require some help. I'd find someone who has these skills/tools and will make what you need out of materials they know will be suitable for the job.

...you have an offer of help from someone above me?

Plexi glass
Make a paper cut out EXACTY the size you need to replace the screen part of your door, minus the space the kitty walk takes up. Silicone the piece in the three sides, and then have the kitty walk slip underneath (leave room for the lip of the walk if there is one)

Take it to the nearest glass place that works with plexi glass.

We had 2 windows cut out this year, to put in the top halves of the windows we put our air conditioners in. It worked great, and was cheaper that I thought. 2 custom pieces cut to size and delivered was under $30, so your piece should run somewhere around $40ish I would think.

We were able to simple exacto knife the silicone at the end of the season, and take the plexi window out, and put the glass one back in for winter. Next spring we will exchange it when we put out air conditioner back in. We rent, and our landlord was very pleased with what we did, and was very happy we did not block off the window with a piece of plywood, like most people around us do. We also live in a very windy area, and the siliconed plexi did just fine.

that's an interesting idea . . . you mean to silicone it into the screen door frame? my apt. would probably get mad, but hmm.

It's better to silicone it into the frame, but either way, it's easy to clean up when you remove it.
Use silicone RTV, not silicone caulk, and it will peel off without marring the finish.

If she uses the frame itself, the door will not be usable for people.... the screen frame would be moveable... and less of a chance the landlord will have an issue. Just a thought. It worked great for what we needed.

The screen frame slides in it's own track, just like the glass door does.

If the screen frame is used and has the tunnel attached to it....then it can't slide open/closed, can it? Everyone will be trapped on whatever side of the door/screen they are on.
If you use an INSERT for the tunnel opening and are able to close the screen and sliding door to IT, then the opening will still be accessible for both humans and cats.
Also...if you remove the apartment screen door and store it somewhere, you can use a replacement (used would work) and slice it till your heart's content. Upon moving out of the apartment, the apartment would still have the nice screen door and the cut one can be removed/junked.

The tunnel doesn't have to be permanently attached to what ever she puts in the screen frame, but using a junk frame is an excellent idea.

Ah! Gotcha! I didn't understand.

I had a similar problem and designed a temporary fix that worked well enough - bought one of these:

http://ecx.images-amazon.com/images/...500_AA280_.jpg

Then I took long thin strips of wood, put them over the bottom edge of the screening, and nailed it down to the deck. The cats didn't try to climb the screen walls, and this worked quite well for a few years, until I was able to afford to do this:

http://img502.imageshack.us/img502/5112/img0240ry3.jpg

Yeah, it's all about the kitties. ;)