The new community manager Yaiden answered several questions about the upcoming Heroes Kindgoms web game. He says among others that although the game will be free, some paying features will be included:

"You can play the game for free as long as you want! You can build your city, hire heroes, play every feature, form alliances with other players and enjoy the game. However if you want to develop your kingdom without any limits, enter the rankings and be a part of the story, you will have to subscribe to HOMM Kingdoms at a small cost.
One of our main objectives is to ensure the game is balanced, that is why we don’t plan to include any item selling system or any other buyable features / specific skills."

One of the most intesting features is the ability to appear next to your friends on the map, to form an alliance or to make sure they are within reach of your archers. The game's official forum can be found here.