Hi. To make an arc simply Click the block tool (shift + b), and press arch in the objects menu (as seen on picture 2).
To make a arch you can walk through:
Make a block. Make it 256x256x128 and press enter. This will bring up a menu. Enter these settings:Press enter. And there you have it. An arch. You might want to rotate it, and if you want to make a complete round arch, make it half the height.
To make a part of a wall select arch and click the Vertex tool (shift + V). And select each little bit of the outer part of the arch, and make it into a sqaure. As seen on picture 3. Now you have a perfect arch walkway.
You can also make a spiral stair with an arch. Make an arch but instead of the other settings write this:That will make each bit of the arch 16 pixels higher than the one behind it.
Play around with it, untill you find the settings you like the most. To make a complete circle you just type 360 in the Arc.
Thanks for reading, HawerWasted