Heroes and Castles 2 Free is actually a standalone that's been adjusted to fit into the free market. The paid version is still very much available, but now players have the option of playing without paying if they so choose. So what's different? For the most part nothing. Crystals (needed to upgrade everything) take a bit longer to earn, heroes are a bit more expensive, and you'll need to watch an opt-in video ad if you want to collect the random loot drops you find during a siege.
If you've been wanting to get your hands on Heroes and Castles 2 but just haven't been able to justify the purchase, well now's your chance to take a loot without paying anything.