Tier 1 is AppGameKit Basic. You don't need anything else to work with this for PC.
Tier 2 is a C/C++ interface to the AppGameKit libraries - for this you need the free or paid version Visual Studio. You have access to vast amounts of powerful code, and compile to a native code executable, not an interpreted file like the Basic produces.
Tier 3 (unofficial name!) uses Pascal instead of C/C++ but works just like Tier 2. For this you need either Delphi or Free Pascal compilers.
In all cases you may need extra things to produce Android and Mac code: Eclipse and the Android SDK (+ Cygwin for Tier 2 but not 3) for Android and a Mac and Xcode to produce iOS and Os X apps.
The main advantage of T2 and T3 is that you can work directly with the host operating system if you know how to do it.
Some people on the forum prototype in Tier 1 because it's faster, and then go to Tier 2/3 for final production.
-- Jim DO IT FASTER, EASIER AND BETTER WITH AppGameKit FOR PASCAL