Defense is one of three phases of Prismata gameplay. The other phases are the Breach Phase and Action Phase (which is composed of the sub-phases Purchasing Units and Using Abilities).
Defense Value vs Attack Value
Defense is a value calculated by the total health values of units with the blocker ability that are actively blocking. Note that units stop blocking when they use their ability or they have been chilled equal to their health. Attack value is simply the sum of the attack of the units attacking.
Defense
During the Defense phase, the player's defense value is compared to the opponent's attack value. If defense value is greater than the attack value the player will be allowed to assign the incoming damage to their blockers. If the attack value is greater than defense phase, the opponent will proceed to the Breach phase and be able to assign any excess damage to the non-blocking units they choose. In the event defense value and attack value are equal the blocking units will be wiped out and the Breach phase will be triggered but as there is no excess damage to assign, the opponent will simply pass.
As the player, you want to prevent breaches by always having a defense value that is at least one greater than your opponent's attack value. In doing so you can use the absorb mechanic to maximize your defense.
Absorb
"In general, absorb is the strongest, most busted mechanic in the game, and you want to be taking advantage of it as often as possible." - Will Ma, Lunarch Studios Founder
Absorb refers to the ability of non-fragile units to take less than lethal damage and then instantly heal. It is a critically important concept to effective defense.
For example, Wall has 
3. When defending, if the Wall takes 2 damage, the health will instantly return to 3 after the defense portion of the turn is completed and the two damage in question becomes absorbed with no effect. Compare to Aegis which has 
5. If the Aegis takes 2 damage, it will not heal and will be left with 3 remaining and therefore the two damage in question has an effect. A defense based solely on fragile units loses the ability to take advantage of the absorb mechanic and is therefore not recommended.
When using a combination of fragile and non-fragile units in your defense, you want to use fragile units in a manner that maximizes your ability to absorb. Taking lethal damage with a fragile unit is acceptable if it allows you to absorb some of the remaining damage with a non-fragile unit. For example, with your opponent attacking for 7 damage you would want to defend with Aegis in front of Wall. The Aegis would take 5 damage, leaving 2 damage that could be absorbed on the Wall.By blocking in the other order, the Wall would die, leaving 4 damage to be taken by the Aegis. The Aegis would survive, but it would only have one health remaining compared to Wall's three health after healing.
In another example, if your opponent were attacking for 3 and you only had a Wall and an Aegis available for defense you are in a situation where you have no chance to absorb damage on this turn. However, defending with the Aegis and keeping the Wall alive will give the opportunity to use the remaining 2 health Aegis to maximize your absorb on a future turn (for example, if your opponent attacks for 4 on the next turn you can take 2 damage on the Aegis and then absorb 2 on the Wall). Clearly it is best to avoid these "no absorb" situations in the first place by having blockers with a variety of health values, but if you are forced into them (for example your opponent is using chill) remember to consider how to best maximize absorb on future turns if you have the option.
An exception to above rule would be when the fragile unit defender has an ability you wish to use, such as Protoplasm's ability to be sacrificed for 
. In Protoplasm's case you might even be able to maneuver your defense such that you can reduce Protoplam's health to 1 (putting it last and tanking 3 damage, potentially saving a defender like Wall) prior to activating the ability.
External Links
- Prismata Game Analysis #6: Offensive Finesse, a video by Argeiphontes. (December 27th, 2014)