In a few maps I've downloaded, it said that there were destrucable cliff faces connecting areas of the map, but I've never been able to find or destroy them. The YR redme says some maps are enhanced with this, but I haven't found any. I'm just wondering how to make them in Final Alery 2.
Also, is it possible to have water passages (like under islands) that can clead to other areas of the map?
For the destroyable cliffs, ask the people from The Project. I believe those maps you got require "The Unofficial Terrain Expansion" and are probably noted with a (TX) at the beginning of the map name. Don't know where the site is now though.
And for you second question I don't think it can be done.
I think ive actually made a map like that without Tx..... i converted it from TS and i could attack a certain cliff on the map but i coulnt desroy it. So you might be able to destroy them without the expansion
You can't destroy them even with the expansion and I can't figure out any differences between the tiles in TS and RA2. All the expansion does is give you a modified FAData that allows them to be placed. To make destroyable cliffs, you need to cheat to some degree and make structures that can sit over a specially constructred cliff ramp and look like a destroyable cliff tile. DZ's mod does this. Water tunnels are possible I believe and will be implementing some in a future release of the TX. You will need the TX to play such maps though.
The way I did it was make a 'structure' with edited cliff images, the used FA2 to place that structure on the map across a ramp leading up a cliff Looks very convincing.
The reason they were disabled in RA2 was because anything could target them, and it looked kinda dumb to see a dog bite a cliff face and destroy it...