Siege mode is a game mode available seperate from the campaign. You play as the Evil army and siege the castle.
The evil hero's level scales with the good hero's level (in siege mode its always 1, but in versus it can change) which also determines starting Dark energy.
Every time you start is a fresh start, Nothing carries over and Heroes do not level. The units you can summon is based on how far into the campaign you are. The objective is to destroy the Keep or the defending hero. The only way to win is to destroy the Keep.
You start on the opposite side of the battle field and next to the Wormwood Tree, The Wormwood Tree rapidly heals you while near it and allows you to switch heroes while near it.
You purchase units and hero upgrades/switches with dark energy. Dark energy is gained every second based on the number of Cultists you have. Your base rate is 1+1 per cultist up to a max of seven cultist.
Every hero has 3 passive abilities unique to them that only affect them that you can purchase with Dark energy along with your choice of 3 out of 9 active skills that will carry over between heros. You can also purchase Health, Armor, or Damage upgrades for the Hero defending the castle.
After the game's over you will receive crystals as the reward.
There are 3 special units only seen in siege mode, they are the following:
Cultist- the evil equivalent of a gold mine, they produce dark energy which is your currency, Max of 6.
Standard Bearer- Enables higher level units to be summoned and increases health and damage of undead units by 20%. Max of 5. These are also spotted in Corte.
Goblin Drummer- Enables you to summon trolls and chieftains and increases Health and Damage of all Greenskins by 20%. Max of 3.
Versus mode is a multiplayer gamemode that is a multiplayer version of the siege mode.
One player will play as the Good side and one player will play as the Evil Side. After a match you gain or lose rating based on whether you won or lost. And the amount you gain or lose is based on your opponents rating compared to your rating.