Can anyone explain what makes ramps usable in Dwarf Fortress?

Solution 1:

Ramps can be somewhat counter intuitive. Here is my understanding of how they work. A unit is standing in the ramp square. It is on the lower level. It has the option to move to one of the adjacent squares on the upper level, when all of these conditions hold:

• The adjacent square is open (units can't walk into walls of course).
• The square under the square that the unit wants to move to has a wall or a fortification in it.

Under certain circumstances you can have a ramp with the square above it blocked. A ramp in these circumstances is useless. (except for certain minecart exploits)

I have some ascii art side view diagrams of this. Let / be the ramp, _ be a square with a floor, but no wall, 0 be a square with a wall and floor, and an empty space be a square that is completely empty. A usable ramp looks something like this:

0000000 ____
_______/0000

Another example is this:

___ ___
___/000

When a dwarf is standing on the ramp square (the /) he can move up to the square just above and to the right in both of these cases and he cannot move into the square above and to the left.

Here are some broken ramps:

___ ___
___/___

Here the wall on the lower level is missing, so the ramp doesn't work.

___ 000
___/000

Here the upper level is blocked, so the dwarf can't get up the ramp.

Hopefully these examples help to clarify the situation.

Solution 2:

Ramps are usable when it makes sense for them to be. Try building some and see what happens instead of fretting over it - the game will tell you if they are unusable, after all. That said, I've tried to simplify:

• There should be nothing directly above the ramp which would block a dwarf. You can't follow a ramp if it leads into the ceiling.
• There should be a space next to the ramp which the dwarf can enter after following the ramp. You can't follow a ramp if it doesn't lead anywhere.