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Start off with an elongated room, like a hallway, where you want your arch to be. Make a Celing height wall through the middle of the room or where you want the archway to be. Thicker arches are better because they seem more realistic than a skinny arch that could barely hold up to weathering.


Add in walls next to it to complete the wall across the room.

Create a brush that is the same width as the center section of your wall. Go to brush > arbitiary sided and enter 16 sides for your brush all while it is highlighted. This will turn it into a flat wheel-like shape with 16 sides.

Select your wheel, then go to Selection > Arbitiary rotation and put in 90 for either the X or Y rotation. To figure out which one you do, does your archway run X or Y on your grid? If X then rotate 90 degrees in the X. If Y rotate 90 degrees in the Y. Don't click Apply, but click OK. Hitting apply will apply the rotation, then hitting OK will apply it again leaving you with what you started with.

Position your wheel, now parallel to your wall, on your wall (so it would do the shiny thing in game) and resize it so it is as thick as your wall as well.

Press the (Picture) button to cut the arch out of the wall. Then press backspace to delete your wheel. You should now have a hole in your wall.

You can now either delete all the lower brushes in your wall or put another block over them below your archway and cut them out as well, press delete to get rid of your block. Either way you should end up with something like this:






