Disclaimer: I am not a Sharepoint developer, but I know my way around ASP.NET fairly well.
I have inherited a simple ASP.NET web forms page (one .ASPX and a code-behind) in our current Sharepoint environment (2003, I think...very old). It doesn't interact with Sharepoint itself at all, only making some simple queries against our AD domain (using System.DirectoryServices) and some third-party web service calls. It needs to use the site master page, though.
In its current version, it is an "application page", deployed in Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS. Apparently, that was the path of least resistance at the time.
My question is whether this is still the recommended approach in 2013, or is there a better/simpler way to do this? If an application page is still the best way, is there a start-to-finish tutorial that walks you through building and deploying a page in 2013?