A combination of original SDKs by Croteam improved to provide better modding of classic Serious Sam games using modern Visual Studio IDEs (but still using MSVC 6.0 compiler).
The entity and game logic is significantly different from what was left since 1.07, which is quite difficult to recreate. However, the engine part is much easier to reverse-engineer. The engine SDK allows for creation of new entity packs, as well as minor patches to the engine itself, like Serious Sam Classics Patch for classic The First Encounter & The Second Encounter games.
This issue is for keeping track of changes to Revolution SDK, which is located at Sources/Includes/EngineRev.