How are transmutation circles built? I've seen Is each transmutation circle unique? (SciFi.SE) where the flame alchemy circle is explained.

Is there any rule to build the transmutation circles? Are there other symbols? What do squares, pentagons, pentagrams, hexagons, hexagrams, etc. mean?
Almost every symbol, transmutation circle, and array in FmA;B have meanings behind them, even the ones who only appear on screen for a few seconds, like Riza's tattoo (here's a link to a really good analysis of it and here's a video where I talk a bit more about it). All the gates shown also have personal meanings to the characters (I also talk about that a bit in the video). Most of the symbols are based on basic alchemy and I found out a lot of things by simply googeling "alchemy symbols"; so yes, I would say that it is totally possible to make your own circles by combining the diffrent symbols.