I was forging recently and I wanted to create a structure that was perfectly circular. It was pretty hard to accomplish, so I thought, "What can I use as an outline for the circle?" Low and behold, I changed to an objective gametype and spawned a spawn area. This gave me the perfect outline for a circular platform/structure. You can also change the spawn area's shape to make a square or a rectangle. The dimensions are changeable, so you can outline many different-sized structures. Oh, and make sure you place the spawn area well above (or below if you're planning to make a floating structure in a certain shape) because the spawn area marker is immovable and will get in the way of what you are placing. Hopefully this will help you out whilst forging.
I looked around and I couldn't find anything about it, so I thought I'd make a thread. Hope it helps ya, Mort.
From the Forge FAQ posted 3 weeks ago. Q. A. Go into Forge mode, change the gametype to Slayer, create a "respawn area", and set it to 'Cylindrical', at the desired circle size. Then use it as an interlocking guide.