Making curved stairs using regular brushes.

This tutorial shows how to create a spiral staircase of 12 steps. Normally in Quake2, each step is 16 units high, so the total height will be 192 units. You can use different sizes of course.
The images are from QERadiant but building curved staircases is something you can do with any other capable Quake2 editor, although some things might look or work different.

Why do I use a 12 sided cylinder? Mainly because it gives the most economical and convincing curve, less sides will make it look blocky and more sides will only complicate things. It's tricky enough so keep it simple.

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<1> Make a brush: 384 x 384 x 16 (for example), duplicate it and place both brushes ontop of eachother.

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<2> Select the top brush and while selected go to:
>> Brush > Abitrary sided... 12 sides

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<3> Now select the bottom brush and diagonally cut it into 4 pieces (2 cuts). These cutlines will serve as guides, you can however remove them later on if they distract you.

<4> [Set Grid 4]. Select the cylinder and snap all 12 vertexes to the grid, staying closest to the orignial shape of your 12 sided cylinder.

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<5> Time to cut the cylinder into 12 parts.
Using the vertex points or corners of the cylinder as a guide, make the first cut from one corner to the opposite corner. Select both freshly cut parts and make the second cut. Repeat this until you have 12 parts, six cuts in total.

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<7> [Set Grid 2]. Align/snap all 12 vertexes (of the new inner cylinder) to the grid, closest as possible to your cutlines.

<8> After that, delete one "half" (5 step parts) of your cylinder. We will mirror the other half later to save some time.

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<9> [Set Grid 2]. Following the shape of the inner cylinder, cut off the seven inner parts. (You'll notice the marked vertexes are slightly off the grid.)

<10> Manually snap those to the grid... again using the inner cylinder shape as your guide. (Don't move the vertexes of your inner cylinder, just the ones of the seven slices.)

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<11> [Set Grid 8]. Duplicate these 5 parts, flip them horizontally and put them in place.

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Select all 12 steps and raise them to their 'up' position. Deselect what's going to be the first step and sink the other 11 steps 16 units. Deselect the second adjacent step and sink the other 10 another 16 units, etc.

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Now if you want to make a round staircase that only goes 90 degrees around a corner you can use parts of your spiral-straircase prefab.
The advantage of the prefab is that you can easily select a corner for every direction, without having to rotate stuff and move things off of the grid.