I saw a picture of a really neat villa in an architecture book, and hopefully someone here will tell me if I need to create something to make it work or if there is a way to do it already.

I'm trying to build a house on a sunken lot (which I lowered to match the terrain at the back of the lot) with a causeway linking the sidewalk (at street level) to the house. I want sims to be able to walk on this causeway to the house. I was able to create it visually using some retaining wall custom content, but my sims won't walk on it. I searched this entire site for a building mod that would make this work, I scoured the tutorials, and I tried to make the causeway out of the driveway as well as columns. But it won't connect to the house. Is there a way to do this, or is there something I can download that will allow this? Or do I indeed need to make something?

(Please tell me that I posted this question in the right place. I figured that people who know something about creating mods/hacks/objects/whatever they're called would know if I need to make something to allow this.)

(Edited for spelling.)

The main problem is that you are trying to allow sims to cross multiple levels without stairs. Here's a thread which discusses this issue and gives some solutions:
http://www.modthesims.info/showthread.php?t=402863

There are two good solutions in that thread: an invisible recolor of a modular stair, or a special fixed stair which has no elevation. Both of these solutions have somewhat odd animations, so try both to see which one works best for you.

The modular stairs will require you to provide an elevation difference of at least 2 clicks while placing the stairs; you can then reduce that elevation difference to 0 clicks for appearance. To change elevations, you can use the constrainFloorElevation cheat, but the GridAdjuster (see my profile) can really help with (nonstandard) 2-click and 0-click elevations. Note that I have no idea whether your retaining walls will accept a modular stair.

Of course, if you'd like to make a 0-height invisible stair with perfect animations, so much the better. I'd download it!

Ok, thanks! I'll play around with all of this and see what I come up with!

I read the thread multiple times and used regular modular stairs in my game to make sure I understood what you were talking about with the different levels. This, of course, worked: the sidewalk is connected to the causeway and my sims will walk over it. However, the stairs go right across it and sims look ridiculous walking over it. That was to be expected. So, as discussed in the thread and as you said above, the modular stairs need to be invisible and there needs to be normal walking animations rather than stair animations. But didn't amiehiggs already solve this problem? The last post shows a sim walking normally from one area to the other, albeit through the stairs. And that problem would be solved with invisible stairs, which is mentioned as apparently already been done. So has this issue indeed been resolved?

I'm not aware of any perfect resolution. The animations are off for both fixed and modular stairs. With the 0-elevation fixed stairs, the sims slide from one tile to the next. With the invisible modular stairs, they walk up the invisible stairs and seem to "trip" during the process, perhaps because they are falling back to the ground.