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Expanding first map
So we learned how to create a basic map containing two cubes connected to each other. Now when you take a look on the toolbox, you will recognize you can add more basic geometry forms to your map than just cubes.
They are pretty much self explaining, just need to adjust properties before subtracting it to map.
Anyway, subtracting adds the BSP geometry to the empty space where we want, but what if we want to add pillars or stairs as geometry in our cube? Subtracting won't make sense because we cannot subtract geometry from geometry. Therefore the left button next to subtracting can add geometry to an existing room. So if we want to add stairs, right click on the stairs icon, adjust properties and move it to desired location in your already existing geometry and click add to apply it.