You know, they're pretty similar, so maybe we should just combine my tunnel idea and the ramp idea. We'll have a ramp, wide enough for wagons (at least eventually) carved into the cliff face and ending in a large tunnel.
When we get around to it, we build whatever fortifications we want into the tunnel- barricades, fortification walls, traps, you name it.
If we feel the need, we build a bridge inside the tunnel, putting spikes, cage traps, or a chute to the cliff face underneath it. This would be almost identical to a cliff-side bridge, except we get more control over where it leads. (Neyvn: Using other Z-levels is alright as long as it's kind of basic, right? Like, a stairway leading to a cage trap filled pit? Wouldn't even need a map for it)
The only concession anyone needs to make with this plan is that we'd be abandoning the U-plan, which I don't think is as good as a fortified tunnel anyway; among other things, we'd shoot our loot off the cliff. As an added bonus, the ramp would take a lot of space, meaning it'd be just about perfect to start on our side of the river and end underneath the waterfall. Doesn't get much more epic than that, eh?
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Obviously, the ramp would be much less steep, but you get the idea. Note the open cave and parts of the downward ramp behind the waterfall.
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The main tunnel would run right under the river, but deep enough that moisture wouldn't be an issue. The ramp would need plenty of overhead room, but other than the initial ramp we'd leave the cliff face largely as it was.
In other words, the ramp would be a spacious tunnel, not an open-air platform. It would still be open on the cliff face side, however.
Also note that enlarging it to allow for wagons would be remarkably simple, since we'd literally just repeat the process next to the initial path.
Well, what do you all think?