A lot of this information is given during the tutorials of the game, but in case you were busy speeding through them to explore the game, or if you're just getting back and need a refreshener this page is made to assist you in remembering how to attack a target, and what the information thrown at you means.To attack a target on the map you have to simply Left-Click it. Granted that your hero moves correctly (since pathing sometimes doesn't do you justice) they will move up to your target, and a window now much different than the one above here will appear.
On the left is always you (4), and the possibilty of an ally. It is always you attacking a target, for you cannot get randomly attacked.
You may only be at most two heroes in the same battle. Fighting side-by-side with another is generally not faster, but easier. You may either invite someone with the manual inviting system (8) which is reccomended if you know someone, and you're planning on playing together, or you may hope that someone eligible for the battle and accepts an invitation from the auto co-op request system (9). The auto co-op request system sends out a mass invite to anyone in the area and eligible for the battle, so if you recieve an invitation through this system and want to join you better do it fast before someone else takes the spot!
As you may only attack, the creatures you attack may only defend (game mechanic wise). The story may state differently of course.
Creatures always belong to an affiliation (5) which gives a general idea of what type of creatures you are going to face. The portrait also show what at least one stack you're facing will most certianly face (in a normal battle, this does not apply for expert and heroic battles). They are also represented by the sprite used by the creature on the map. You may also press the details button (6) to know exactly what you're up against, but this costs 25 hero seals for one time uses, or you may purched Enemy Detail Licences of various timelimits to see this at a pre-purchased cost for a limited time (3, 7,or 30days).
If you are unsure of your ability to face the cretures, and fearing you will lose a lot of creatures (troops) you may use the auto-battle option (7). Like Enemy Details this costs hero seals and a timed licence for free auto-battles during it's time can be bought. Using auto-battle still depletes some (e.g. 5%-10%) of your creatures indicated when hovering over it with your mouse, so make sure you know it is worth it if you want to take this option. If your decide to take it: check the box.
Perhaps the first thing you look at when you enter this pre-battle stage is the threat level (1). This is an approximate indication of how hard this encounter will be base on your battle power (3) compared to yours. Battle power in itself is an quantification of spell power and army power to approximate how strong one is. There are 5 threat levels, and an achievement associated with each asking you to defeat a certien number of encounters of this difficulty. The levels are; Easy, Average, Challenging, Sever ,and Deadly. There are a few different threat "types" that don't follow this threat level system; Boss, Siege, and Event, and these three are aleays displayed with purple ("Deadly") icon regardless of their battle power compared to yours.
Moreover there are three different types of battle-difficulties (2):
The star in the middle is Normal Battle. All battles have a normal battle difficulty. This is generally used while progressing through story, doing dailies or facing bosses of all types. It is particualarly useful while doing dailies which as you to kill a certian type of creature since you can spot with the portait which type you are facing and can guaratee you will get progress from that battle.
The skull on the left is Expert battle. This option is only avaliable to battles that are not Boss, Siege, or Event. Expert battles grant slightly better rewards than Normal battles, but are also fought because there are town quests which require you to defeat a number of expert battle of a certien creature affiliation for a generally large reward (in Wood, Metal or Gold).
The crown on the right is Heroic Battle. These battles are unlocked on level 30 and can be progressed through 13 tiers, and each tier makes them a little bit harder. The progress is long and costly in Gold, Wood, Metal and, Dragonsteel, but luckily battling on Heroic Difficulty may grant you Gold, Wood or, Metal as reward, and will always grant Dragonsteel (5 per battle, +15 if you're battling on your highest tier). Heroic difficulty is also the source of many set items and unique items.
Finally you get to the point when you have to make the choice. Retreat (10) or Attack (11)! If you retreat, nothing happens, but if you attack you start the battle.
When in a co-op preparation the Attack button switches to a Ready button, and the battle is only started if both participants have pressed Ready.