Arwin and xbd, I'm trying to build another deck now (for wife). It would be great if you can give some tips.
What kind of tips are you looking for? This thing is huge.
Depending on the kind of board you play on and so on ...
But here are some general tips, for the standard board:
this game has the following basic premises:
Elements
The cards not very surprisingly take to a certain element (biolith excluded). The cards that belong to an element are obviously a lot better off when placed on a field of their own element - they will typically gain a lot of health on their home turf and are therefore harder to kill. Sometimes they also gain other abilities.
Sometimes though they also have some extra abilities when they are placed on any other field. The ice guards for instance, allow you to draw an extra card when you summon them on anything but water.
Sometimes, however, they also lose power if they are put on a field they really don't like. This isn't often stated outright, so it's important to keep an eye on. A typical forest creature will lose 2 health when placed on an earth field, and I think vice versa. But even that can be a good thing sometimes - there's an elf creature that goes berzerk when her power goes down to +1, where she gains a dodge attempt (a physical attack or counter attack can miss) and her attack goes up from +1 to +3. So as she has +3 health by default, if you place her on an earth field, she goes to +1 immediately and becomes an agressive attacker. She only costs two, so very valuable.
There are also cards (magic and creature) that target all creatures of the same element. One that restores your health for instance, can be very powerful that way - already at a cost of 2 you gain 3 health for one creature, but if you have three creatures of the same element on the field, that gains you 9 health in total, basically.
Symbiotics
Some cards have a strong symbiotic quality. The towers in the screenshot above offer +2 protection to all your adjacent creatures, which is very useful. Verzan footsoldiers gain +1 attack if there are two of them, +2 if there are three of them, etc. Some Ninja's gain invisibility from other Ninja's. Lots of stuff like that. However, when you put them in your deck, you really can't count on being able to use this quality unless you put in several of each in your deck.
Orientation
Many cards can only attack the field in front of them, and are extra vulnerable when attacked from behind. Especially in the beginning of the game, unless you can place them on their own element, these cards are typically very easily killed by another card. Some cards can attack multiple fields around them though, and that makes these cards much more useful to place on the field first - when attacked, they will more likely to be able to counterattack, which doesn't cost you mana (unlike your opponent, or when you yourself attack using that creature). Quickness helps there too, as you get to counterattack before the other attacks. An extreme example of where this is important - if you both have 1 health, then the one who attacks first wins. If you have a creature that can counterattack from multiple directions, that's where orientation counts. Conversely, you can also cover a creature's weak spot by strategically placing another creature next to / behind it.
Even more interesting in this game is changing a character's orientation. There's a 0 cost beguiling spell that's an essential part of my deck and I already know will never leave, and if I ever get more of them, then I'll add them to my deck immediately
(though I don't know the max for them, and you have to think about keeping enough creatures in your hand too). Whenever a strong creature that really worries me appears, I can often render him completely useless and in some cases even more vulnerable by turning him around 180 degrees. The elven archers are very effective with that - they can only attack at range, but when they hit, they turn around their enemy around 180 degrees. Very useful.
It seems that you can actually turn a card (though I've only recently just read about this and never tried it yet), which costs you the activation cost each time you turn the card 90 degrees. But if a card is really powerful, the activation cost is also typically high, so it's still extremely painful to have the card oriented wrongly.
Mana management (and some basic deckbuilding)
You definitely need your basic low cost cards. They are really useful in taking a hold of fields (after all, winning = hold five fields), and keeping pressure on your opponent in that area can be vital. But these low cost cards have a lot of variety. Very useful for instance are low-cost ranged cards, like the fire magi wich can attack any field. Especially if you can lock that one up on a fire field in a corner guarded north and east, that's very useful.
Low cost cards also can be used with a little less discretion - whenever a card dies, you gain back 1 mana. So if the card cost 1 mana to cast, then you lose very little. Generally simple value calculations for actions for me involve thinking about how much health points I gain, or take away from my enemy, at the cost of how much mana. Also, low cost cards you can activate with some spare mana, whereas high cards are typically really expensive to activate (for an attack).
Some more expensive cards can't come into play unless the majority of fields are occupied with creatures. When you think about using an expensive card, keep that in mind. Also, if you run out of time in your turn, in ranked matches as a penalty you have to discard the most expensive card in your hand, so be careful there.
Also, when playing against an opponent, make sure you know the card you are fighting (use the status card). I've seen plenty of people not think this through.
Area of Attack
When attacking there are also a lot of different things to think about. Attack area I've mentioned above a few times already, but here's a few more points.
Some cards can attack multiple fields at once. If you use them, make sure you can use that ability, because otherwise they are expensive to use. Thinking about area of attack is about as important obviously as orientation.
Some cards also attack your own. I've seen a fair few people make the mistake of putting a nice three headed dragon down, which attacks +5 if there's just one card east, north, or weast of it, but that goes down to +3 if there are two, and to +1 if there are three creatures next to it. But most importantly, if there's one creature in front of it, and an allied creature next to it, then it will take three off the health of the creature in front, but also 3 off the health of the creature beside it. And that's a painful surprise, I can tell you!
(seen it happen plenty)
Magic cards
These cards are often extremely useful. I don't have that many yet in my deck, because I just don't have that many cards yet.
You also shouldn't have too many of them in your deck. But there are some very useful ones - obviously the orientation one I've talked about, but also others I've seen used against me some of which for instance give each player as much mana as the opponent occupies fields. So especially if you're really behind that gives you a great mana boost and allows you to strike back hard.
There's lots more tips, but this is a good start I think.
