Its an RPG style card game, with the ability to gain levels, earn gold and complete quests. The player has a choice of 4 Classes: Assassin, Druid, Sentinel, and Pyrelord; each having their own strengths and weaknesses. Additional classes can be unlocked by gaining levels and purchasing the class with the use of Skill Points.
The story takes place after Dawn of the Dragons, another web-based RPG developed by 5th Planet. You're from a royal family known as Kasan, you were forced out of your home in West Kruna, by the current king, Crenus. After moving to East Kruna, un-expected events cause you to return home, and that is where the story begins.
CotD is currently in BETA, and has only been online for a few years. Even with that being said, CotD has a very large population, a very active and helpful forum, and dedicated developers and administrators. The game Updates weekly on Tuesday 00:00:00 GMT but can update at a different time, which the developers will announce ahead of time.