Sounds similar to, but not exactly, any of the following:
Rambit CB, standard CBA but kills earn kings, use em to get more castles or special units. A new version ripping off this format exists on DE, called CBA KingTe.
CBA front towers, it’s CBA but FFA and random units. You can swap yours or your opponent’s units with kings, and there are kings for upgrades.
Archers Hellfire, archers spawn and get stronger in middle. Get better archers for kills and monks for special powers.
Path blood, earn sabs with kills and use em to unlock better units in your choice of paths. Units get stronger in middle.