When your hero is 1 tile away from a monster, you have about 1 second to click on the monster to attack first or let the monster attack you. It is recommended to attack first when you have more than one monster surrounding your hero so that you can pick the ones you want to attack.
After a fight, the monsters will be frozen for 1 second (or less) to give you a very short time to pick another one.
If you don't attack, you can choose to let the monsters attack your hero or run away. You may escape for a while but the monsters will run after you. Every monster has a "field of vision" parameter which describes how far and how long they will run after you. If you move further than this FoV value (in absolute tile distance), they will stop running.
In turn-based fighting you have 5 actions points to use before your turn is over.
Tapping the attack button ends the turn (5 action points), unless your hero has more than 100% attack speed. In that case you will be able to attack 2,3,4,5,... times if you can click fast enough.
Casting a spell costs 3 points
Drinking a potion costs 2 points
Changing weapon costs 1 point
Instead of 5 action points you have 5 seconds to perform as many actions as you can. Tapping the attack button ends the turn, unless your hero has more than 100% attack speed. In that case you will be able to attack 2,3,4,5,... times if you can click fast enough.
It takes around 1-2 second to cast a spell, 1-1.5 seconds drink a potion or change equipment.
The value your hero has in shields (defense) and swords (attack) represents the number of dice that your hero will roll for defense or attack. The faces of the die will determine whether or not you score 1 point for that die.
Your hero's max attack/defense points for that attack/defense phase will be based on the number of dice you throw. Inflicted damage = attack - defense. Each die is a HeroQuest die and based on who throws the die (monster or hero) the odds to score 1 point for that die will be different: black shield for monster's defense vs white shield for hero's defense.
Also, if your hero had a defense score above 0, it will pick one of his armors at random (including the ones with no durability) and decrease the defense counter for that armor. An armor with 0 defense counts will be rendered unusable and your defense/attack handicap will be reset as if you were not wearing the armor.
Each weapon has a weight and using them will impact your heroes' stamina. The more you use your weapons, the faster your stamina will deplete. When your stamina runs out, you will be left fighting with your fists. Do not worry though, your stamina will replenish while exploring the dungeon and at every turn of the fight.
The amount of stamina a hero starts with depends on its class.
The more mind points you have the more mana you will get - typically 10x mind point (value configurable in misc_values.json). The required mind value of a spell also represents the number of mana needed to cast it. So with 1 mind point, you can typically cast 10 spells of level 1, 5 spells of level 2 and so on. When your mana runs out, you can't cast spells until it's replenished. Do not worry though, your mana will replenish while exploring the dungeon and at every turn of the fight.
If you are tired of whacking the attack button and would like a more refined combat system, you can also click directly on the picture of the monster attacking you. Being the head(s), body, arm(s), leg(s), etc. Each zone has different effect when you hit, and you have different chances to hit each zone (turn on the "fight stats" option in the extras to see the percentages).
Head: the hit will be 3x-5x stronger and the monster may lose a mind point permanently
Body: equivalent of hitting the Attack button
Limbs: being legs, paws, arms or tentacles, it will reduce the attack and defense points of the monster permanently or not
Some monsters have weak spots, you will learn which one by trying them out :) This definitely brings more depth and strategy to the fights specially against bigger monsters - try fighting a Crystal or Mithril golem and let me know how it felt!
From time to time, you will notice that monsters that you are fighting gain a "Gang-up Bonus" (displayed at the bottom left of the monster on the fight screen). A gang-up bonus is applied when a monster is fighting with you while other monsters are around him/you. The more monsters around, the higher the bonus.
Each monster around is adding +50% (may vary from update to update) to attack and defense points of the monster that you are fighting. So if you are planning on attacking the bigger monsters first, think again as if a big monster is surrounded by 8 goblins, he will be extremely strong and you should take care of the minions first :)