Defense Rolls

One of the first variant rules I added to my games was the Defense Rolls, published in the Unearthed Arcana: Variant Rules. Essentially, instead of the GM making attack rolls against the players' static AC, the players make defense rolls against the GM's static attack save DC. Implementing this is easy.

Calculating Defense Bonus (DB)

Armor class is replaced with Defense Bonus. This bonus is calculated simply by players taking their AC and subtracting 10. If a player had an AC of 16, they would have a DB of +6.

Calculating Attack Save DC

Attack bonuses you find on monster cards are similar converted to static values, called the attack save DC. To calculate this DC, simply take the listed attack bonus and add 12 to it. For instance, a B1 battle droid's blaster rifle attacks with a +4 modifier. Instead, that droid would have an attack save DC of 16.

Making the Roll

Instead of the GM making attack rolls, players make defense rolls by rolling a d20 and adding their Defense Bonus. If the roll meets or exceeds the attack save DC, the player successfully defends. If a character fails the the defense roll, the attack hits.

If the attacker would normally have advantage on the attack roll, you instead apply disadvantage to the defense roll, and vice versa if the attacker would have disadvantage.

If the defense roll comes up a 1 on the d20, then the attack is a critical hit. If the attacker would normally score a critical hit on a roll of 19 or 20, then the attack is a critical hit on a 1 or 2, and so forth for broader critical ranges.

Similarly, a 20 on the defense roll would automatically be a successful dodge, regardless of the attack save DC.