Back in September of 2017, D&D released the Tortle Package on the DM’s Guild in conjunction with the Extra Life event to raise money for Children’s Miracle Network.
Included in this class was a mechanic called Shell Defense.
It’s broken.
Maybe I shouldn’t say that. Like many things in D&D, it is situationally good. That in itself is a sign of good design, but not enough. Especially not enough here.
The problem with Shell Defense is that it is inferior by a basic game mechanic: Dodge. Racial bonuses should provide a greater benefit than players can realize from the basic rules. Especially something as iconic as a turtle hiding in its shell.
How It Works
The Shell Defense mechanic provides as follows:
Why It Doesn’t Work
Being prone gives melee attackers advantage, which counteracts the Armor Class bonus. At the Tortle’s base 17 Armor Class (AC), using Shell Defense is worse than doing nothing against a melee attacker.
The only practical way to increase Tortle Armor Class is by using a shield (+2 AC).
- The Shield of Faith spell will increase your Armor Class (+2 AC), but it requires concentration. Using a spell that requires concentration to boost your Armor Class obviates maintaining concentration on a spell that actually does something while you’re shelled up (more on this later).
- The Haste spell will increase your Armor Class (+2 AC), but it’s a silly thing to cast on a character that will be foregoing all of its actions. It also requires concentration.
- The Mage Armor spell does not stack with the Tortle’s natural armor. The Tortle would need a Dexterity of 20+ to surpass its base Armor Class with this spell.
- The Barbarian and Monk’s Unarmored Defense do not stack with the Tortle’s natural armor; the rules require the player to select one method of calculating Armor Class.
- The Fighter and Ranger’s Defense Fighting Style do not stack with the Tortle’s natural armor, since the fighting style requires the player to be wearing armor.
Boosting your Armor Class to 19 will allow you to achieve a marginal damage/round benefit over doing nothing. However, this benefit will erode against higher level monsters, and Shell Defense will again fall behind doing nothing in damage reduction.
Consider this: the baseline for comparison shouldn’t be “doing nothing.” The rules already allow you to burn your action to achieve a defensive benefit with the Dodge action. In melee, you’re better dodging.
Example: Level 1 Attacker; 16 STR; 2-Handed Longsword (+5 attack, 1d10+3 damage):
Take a look at the far left of the graph. That’s where you will see the comparison at 17 Armor Class. As you can see, you would actually be taking more damage/round if you used Shell Defense. If you boost to 19 Armor Class, you can achieve a small benefit from Shell Defense. Alternatively, you could cut the damage you’re taking by one-third using the Dodge action.
When It Does Work
Shell Defense works at range, where being prone is actually a benefit:
Example: Level 1 Attacker; 16 DEX; Longbow (+5 attack, 1d8+3 damage):
So you can hunker in a bunker under your shell. But you cant do anything else. So when is this useful?
If you need to run, you should just be running.
You’re going to need a reason to stay in battle (at range), and a method to remain effective without taking any actions. Enter the spellcaster maintaining concentration. Shell Defense works especially well for maintaining concentration, because it grants advantage on Constitution saving throws.
However, unless you’re at the end of your spell slots, your party is going to be better served if their Tortle spellcaster is actually using their action every turn to do something useful. For example, dispatching the very foes posing the ranged threat. A ranged damage cantrip is often going to be a better use of a turn.
Offense can be the best defense. Over a long enough run, attacking becomes a better defensive strategy because eventually ranged attackers will crack the Tortle’s Shell Defense, if our reptilian friend decides to stay shelled-up forever.
Dodge vs. Shell Defense Comparison
Action Economy
| Shell Defense | Dodge | |
|---|---|---|
| Time to Enable | Action | (Bonus) Action |
| Time to Disable | Bonus Action | None |
| Bonus Actions | No | Yes |
| Reactions | No | Yes |
Saving Throws
| Shell Defense | Dodge | |
|---|---|---|
| Strength Save | Advantage | Normal |
| Dexterity Save | Disadvantage | Advantage |
| Constitution Save | Advantage | Normal |
Run Your Own Calculations!
You can calculate the effectiveness of Tortle Shell Defense against various foes by using the calculator linked in the heading of this section. Here are some more test cases:
Example. Tortle vs. Goblin:
Example. Tortle vs. Bugbear:
Example. Tortle vs. Ogre:
Example. Tortle vs. Young Red Dragon (Claw):
Melee Damage Reduction per Attack (Summary of Test Cases):
| AC vs. Opponent Modifier | Shell Defense | Dodge |
|---|---|---|
| 17 AC vs. Goblin | -0.1 | 1.5 |
| 19 AC vs. Goblin | 0.4 | 1.3 |
| 17 AC vs. Bugbear | 0.0 | 3.0 |
| 19 AC vs. Bugbear | 0.8 | 2.8 |
| 17 AC vs. Ogre | -0.5 | 3.7 |
| 19 AC vs. Ogre | 0.1 | 3.6 |
| 17 AC vs. Dragon Claw | -0.9 | 4.0 |
| 19 AC vs. Dragon Claw | -0.8 | 4.3 |
| % Chance Opponent Crits | 9.75% | 0.25% |
How to Fix It
Good fixes for game design problems are elegant but simple. Elegance is fitting the narrative of how the mechanic works without changing its spirit. Simplicity is the ability to state the rule in a brief sentence. In achieving simplicity, it’s important not to paint with too broad of a brush. The fix should address only the narrow scope of the problem, else it may upset the balance of other design elements.
The thing that breaks Shell Defense is the fact that being prone grants melee attackers advantage. We don’t want to remove being prone, because the Tortle should be on the ground if it’s in its shell. That would be an inelegant solution. Let’s look deeper.
Why does being prone grant advantage to an attacker? It seems to be that a defender on the ground is less capable of protecting themselves, and is more prone to having a weakness exploited (i.e. critical hit). However, this is not the case for the Tortle, who is in its best defensive position and covering its weaknesses while in its shell. Simply stated:
“While in your shell, attack rolls made against you do not have advantage normally granted by the prone condition.”
You could rule that attackers against a Tortle in its shell never gain advantage under any circumstance. However, that raises concerns that it may begin to interact with other design elements. Make sure to keep the scope narrow; and carefully consider any such sweeping changes.
ThinkDM 
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