Yes, Balgin has a correct general idea. The main purpose in deck building at the beginning is to use the most interesting abilities/spells/etc. Then if you are going to create a powerful balanced deck - you should refer to different points.
I will try to show you what I do when creating a good deck. Cannot definitely say that my tip would be the most optimal, but my already created decks eventually turned out to be pretty good and competitive.
I use the following idea to create a deck:
1) Choose your style
I choose my preferred playing style. I.e. my #1 is defensive playing style based on fighting off the opponent's early pressure, control, and counter-attack.
Thus, primarily I have to choose the Hero who will be the best option for that. Among City of Vallor heroes, the first obvious option is Khan'Dir as a heavy dangerous tank (but Thibaut is also not bad in this as he has plenty of useful control skills). If to look through Imperial heroes, my first pick is Pontifex Conrad.
2) Pick the Hero cards
The next step is to choose affiliated with hero cards. For instance, Khan'Dir has powerful personal relics. Pontifex, for his part, has his own "security" Morgen Flagellant who is veeery strong in link with the Hero. Almost in every match I played as Pontifex, Morgen almost everytime made a big impact whether he fought back or attacked.
3) Plan the strategy on the Battlefield
That style of play requires special work on placing terrains on the Battlefield. The most important here is to use 3-4 Rocks in order to isolate the enemy from straight multi-flanks intervention. And when the enemy tries to move his big army through one "between rocks passage" - you can use Magical Spire behind their back to close "the rabbit trap".
If you use rush decks you have to take it into account and try to avoid such traps. I.e. you can prevent it by placing a Forest on the spot earlier than your opponent, use terrain removals, and so on. For Vallor it's preferable to make an attempt to place one of their Ruins near to the opponent's Battlefield side (without giving them the opportunity to use advantages of those Ruins) as characters can travel between empty Ruins and it's an easy way to keep pushing your opponent, reaching his side very quickly.
4) Create an army
While building your army, keep in mind that there are a lot of synergies between class characters (guild/orders) and other characters (for instance forces of praetorians and oronox riders, or undead race). In this, I cannot recommend something in particular as it is all up to a player.
Firstly you should pick some key characters that will be the central figures for your game plan. Take into account their playing cost (3-5 buildings) and then gather additional characters. In general, it's ok when the deck is based on 6-8 buildings.
5) Support cards
As in 4), it's also a bit subjective. Control/Defensive decks should pick removals, heal, and so on. Rush or Midrange decks can pick Strength boost, movement increase and dealing damage cards.
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As it stands, we created a lot of decks and archetypes and still, there is no flawless option that will be dominant over other decently created armies. Too many factors are up to tactical skills.
P.S.: once again sorry for taking so long to reply to you as I couldn't figure out some troubles with my account