Elemental Kingdoms is a complex deck building game in which you assemble a deck of up to 10 cards, supplement them with up to 4 runes, progress through maps and dungeons, and partake in one of multiple events, ranging from daily arena battles to other miscellaneous scheduled events.
Main Screen:

The cards themselves each have up to three abilities, and some cards seem drastically overpowered compared to others, but the EK development team has done a relatively good job of balancing things out. In this game, even the strongest cards can be beaten, and can also be bad additions to a given deck. One of the big draws to the game for me is that it's less about the cards themselves and more about how you can create one or more decks tailored to what you want to do. The abilities of the cards contribute to the decks overall effectiveness, and the runes have additional abilities (and requirements) to supplement the deck.
Unevolved Card:

Evolved Card:

Rune:

Deck Screen:

Each card has a cost associated with it, and as you level up, you have an increasing total deck cost. The cards and runes can both be leveled up to level 10, and recently, evolutions were introduced. Evolved cards can go up to level 15, and gain a fourth skill, but they also increase in cost. There is also a high gold cost to level up cards.
There are three major types of currency in the game, all of which can be gotten for free. Gold, Fire Tokens, and Gems. Gold is obtained through day to day mapping, mazes, and events. Fire Tokens have multiple ways of obtaining them, but the major ones are through completing achievements and some events. Gems are obtained through events and by purchasing (optionally, of course) with real money. As far as uses go, Gold is used for leveling, evolving, and purchasing packs to get new cards. Fire Tokens are used for purchasing cards (although it's best to stockpile these and save them for events that give bonus cards). Gems are the bread and butter. They are used to reset mazes, get more energy outside of the standard regeneration, and a myriad of other event-related options. They can also be used to purchase packs of cards, although this is arguably the worst way to use them, especially as a free player.
Store:

Energy is used to progress through maps and mazes. You start with 50 energy and you gain it back at a rate of 1 energy per 10 minutes (save for one window of time where it regens slower). There are other ways of getting energy back faster, and there are two times throughout the day where you get 20 bonus energy. You can have more than 50 energy at a time, but you stop regenerating energy when at 50 or above. As a result, unless you're trying to save energy for some reason, it is a good idea to always stay under 50 energy so you can always regen it.
Sample Battle (Losing in the picture, but ended up winning):

The game has implemented Clans, which are essentially groups of players that can partake in a weekly event called Kingdom Wars, which consists of the clans within each of the four "factions" battling against each other in a territory-capturing type of scenario.
The EK development team is an affiliate of Perfect World, and they do a good job of keeping regular events coming. As with most games, it gets bugs, and the team has been very receptive to feedback. They've also done a relatively good job of fixing the bugs. The game itself also has a fairly in-depth tutorial and information system that details some of the events they have, as well as the basic functions of the game.
Within the game, you can create an account or play as a guest. When you play as a guest, the data is saved on your mobile device, while creating an account will store it on the EK servers. Playing as a guest is fine if you're just trying it out, but I highly recommend creating an account first, just so you don't lose any progress if data gets deleted from your phone. I believe if that happens, you can reach out to their support team and they can get it restored for you, but I haven't tried.
There is a chat feature within the game, but it's fairly limited. As a result, many of the players, myself included, use the chat app Line. There are several benefits to this, including easier communication and access to information.
Even with all of the events, the game itself doesn't take up a ton of my time, which is nice. It is very possible to stay competitive without having to babysit your phone, and you can do so even as a free player. A lot of the cards you get do come down to luck, though, which means there may be times when you get frustrated and feel a bit weak. Don't despair - this changes, and can change very quickly!
I'm probably forgetting things and there are many more nuances to the game, so feel free to ask questions about it. You're welcome to post them here or if you are interested in actually trying it out, you can contact me on Line.
Below are my in-game details in case you do want to check it out and my user ID within the Line app is cubedcodemonkey.
Server: Destiny
In-Game Name: TheDragonReborn
Clan Name: Taveren
Invitation Code: 456z5w
Here are some friends - you might recognize the names (Nikolita is Shadowjack, the others should be obvious, and Lots is slacking a bit):

One note about the invitation code. There is somewhere to enter it, but I honestly forget where. I know a couple of the people I have gotten to play the game were unable to find where to put it. I believe it is upon registration, so if you start with a guest account first, you might not get the prompt to enter it, but I am unsure. If you want to use my code, you're more than welcome to, but if you don't, that's no big deal either!
I encourage you to check out the game, and if you like it and want to continue playing, or even just have questions, add me on LIne and when you reach level 18, you will be able to request to join my clan!
EDIT:
I just realized I should clarify something. I've mentioned staying competitive with relatively little time and without paying. I don't mean that you'll be pulling top ranks in arenas or events easily that way. You will, however, still be able to get strong cards to improve your deck, and you will see progress.