Many people will tell you not to use carve
carve
and they're generally right. It's easy to cause terrifying buggy stuff with it. If you have a block and you want to shape it, I suggest using the clipping/slicing
clippingtool
and vertex
vertextool
tools. Use the first to cut things up into smaller bits, use the second to shape those bits. It takes a bit of practice, and not all shapes the vertex tool lets you make are valid. (Turning a cube inside-out in a certain way, etc.)
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1. Try quitting Hammer, the Source SDK, and Steam, and then restarting them. (In the reverse order, open hammer from the Source SDK menu.)
2. I do not think there is any way to make a donut hole in a cylinder without effectively turning the cylinder into a bunch of shaped and grouped connecting chunks. This is because Hammer brushes are built around convex ([ame]http://en.wikipedia.org/wiki/Convex_and_concave_polygons[/ame]) shapes, shapes which don't have "dents" in them.
So arch is probably the way to go. Remember that when it comes to the final product, there's no performance hit to having two faces which perfectly meet each other, like between all the arch segments.
3. Overlapping blocks are... Well, they're nothing
technically wrong with them, but generally speaking try to avoid doing so, especially where players might see it. (There are some cases where it's easier to let them overlap than to try to re-architect everything.)
Some brush-
entities will have extra restrictions (ex. areaportals, hints, water.) Special mention must be made of overlapping sides with different textures: If you have differently-textured brushes making up a single flat floor, ensure they don't overlap on the top. If so, then the engine may make the two textures "fight" and flicker since neither one is truly "in front of" the other.