You could use editors of all kinds, even Nodepad. But you should know all the specific master pages characteristics, like @ Master
directive for example, because you should write all the code by yourself. Here is an article
that discusses in details ASP.NET Master Pages. Also, if you want to use code-behind, you have to compile the files into a DLL. As you may, know ASP.NET searches for the DLL files in the bin directory of your root folder.
Hope this helps.
the Telerik team