I'm working on expanding a simple water stream based mob grinder, but my brother wants it to not get in the way of the view, so I was considering building a scale-model-ish replica of the Citadel from hl2 around the grinder, but I can't figure out how to build the triangle.

Could you send a picture of the triangle you are trying to replicate?

If you have the coordinates of the 3 points of the triangle, get the slope of the line between coordinates by googling "slope calculator" and plugging in the coordinate x values and z values.

Once you have that you have a pattern or value to follow. For example, if the slope is 2, you would build 2 blocks up, then one to the side. If it is less than 1, like a fraction such as 1/2, you would build 2 blocks to the side, then 1 up. If the value is not easy to work with(i.e. something like 3.5) then you would need to multiply it until it is a whole number. For example, I would multiply 3.5 by 2 to get 7. Now take the number you multiplied by. That is how wide the pattern is. So, in total it would go 7 blocks up and 2 to the right. Now you need to balance blocks so they are roughly the same. For this one, I would split it into 3 and 4. It might look unstraight or zig zagged when building it, but if it is a large structure it looks fine when viewed from a distance.
Hope this helps!

I'm working on expanding a simple water stream based mob grinder, but my brother wants it to not get in the way of the view, so I was considering building a scale-model-ish replica of the Citadel from hl2 around the grinder, but I can't figure out how to build the triangle.

Could you send a picture of the triangle you are trying to replicate?

If you have the coordinates of the 3 points of the triangle, get the slope of the line between coordinates by googling "slope calculator" and plugging in the coordinate x values and z values.

Once you have that you have a pattern or value to follow. For example, if the slope is 2, you would build 2 blocks up, then one to the side. If it is less than 1, like a fraction such as 1/2, you would build 2 blocks to the side, then 1 up. If the value is not easy to work with(i.e. something like 3.5) then you would need to multiply it until it is a whole number. For example, I would multiply 3.5 by 2 to get 7. Now take the number you multiplied by. That is how wide the pattern is. So, in total it would go 7 blocks up and 2 to the right. Now you need to balance blocks so they are roughly the same. For this one, I would split it into 3 and 4. It might look unstraight or zig zagged when building it, but if it is a large structure it looks fine when viewed from a distance.

Hope this helps!