Missions

Missions are the primary "player vs enemy" (PVE) game mode. They are set up across multiple maps in the missions tab of the game, which is the first tab in the main navigation interface, which is, in turn, found at the middle bottom portion of the screen.

They require a variable amount of "mission energy" to participate in.
Energy is earned free over time, at a rate of one energy per minute.
Missions energy starts out at 200 and numerous factors can boost your maximum energy (joining a guild will give you a 1 energy boost for every member in the guild, and many store purchases can upgrade your energy further).
Missions are fought in battle mode, where the active player goes first
They are typically progressively harder and cost more energy than the previous.
Each mission can also be replayed an indefinite number of times, also progressively harder for the first ten successful tries.
A 100 gold bonus comes on the fifth successful attempt, and a separate 200 gold bonus is earned on the tenth successful attempt.

Storyline missions

Storyline missions are found in the missions screen and are numbered consecutively.
Developers periodically add 12 missions to the main story line, whose mission count currently stands at 130.
Sometimes, some special objectives (such as mutant missions or side missions or legendary quests) are only unlocked after defeating certain missions.
These missions earn players one common card for each first successful attempt in addition to gold
Some special missions earn higher rarity cards.

Another type of mission is the event mission.

Event missions

Event missions are limited time missions found on the map, typically at the very final page of missions (right-most) or by clicking a swirling red portal.

Each event mission has 4 legs/phases, each one coming about 2 weeks apart.
Each leg has a set of missions, with the last mission being a "boss" mission. These boss missions earn a rare card for the first successful attempt, and earn an "epic" or "legendary" PVE reward card on the tenth successful attempt.
Unlike storyline missions, event missions can be played up to 50 times for the "boss" mission to earn a second copy of a "epic" or "legendary" PVE reward card.
Apart from cards, these event missions also earn guild points (or diamonds or gems in the vernacular). These guild points are used to purchase fortresses, which are deployed in raids and wars and daily guild quests.

Event missions are "limited time only" missions which can only be completed over a fixed duration, typically one event cycle lasting approximately 6 weeks. Once it is over, a player can no longer progress thru it and must wait to participate in a new set of event missions. Developers made the energy requirement of each leg of the event mission to be able to accommodate players playing and completing each mission 10 times or 50 times (as the case maybe to earn maximum rewards) and still do 5 attempts of guild quests daily.

Each leg of event missions reward a different card:

leg 1: previously released PVE Epic
leg 2: newly released PVE Epic
leg 3: previously released PVE Legend
leg 4: newly released PVE legend plus other base/PVE rewards

The final leg of event missions are played on a special game mode called "campaigns", where a player selects 15 cards and one commander to use across 7 consecutive battles. Cards lost at any stage of the campaign are no longer usable for that attempt of the campaign. Successfully completing the campaign unlocks a higher difficulty tier until 3 tiers are unlocked, and each difficulty level awards cards or card packs. These time-limited missions are not related to the main storyline, like the permanent side missions.

Side missions

Side missions are another type of mission which is not part of the main storyline, but are important in that they reward higher tier cards, like event missions. Some are extremely difficult, like mutant missions, which award cards that are otherwise unobtainable in the game.