Simplified D&D (SD&D): the Castles & Crusades rules modifications to 1st edition Advanced Dungeons and Dragons.

Using these simple on-the-fly modifications, any 1st edition AD&D module can be played using a streamlined ‘d20-like’ system.

Armour class

The Armour class in SD&D is the reciprocal of the armour class in AD&D.

To generate the SD&D armour class subtract the AD&D armour class from 20.

AD&D armour class 3 becomes SD&D armour class 17.
AD&D armour class -2 becomes SD&D armour class 22.

Bonuses to dice rolls based on characteristics

There are two types of characteristic bonus that can be applied to d20 dice rolls.
1) Primary bonus.
2) Unusual score penalty/bonus.

1) Every character is defined by a numerical score against six characteristics: Strength, Dexterity, Intelligence, Wisdom, Consitution and Charisma.
You can specify any three of these characteristics as being Primary. That is: the particular characteristics that more especially personalise the character over and above their class/profession.
Every class specifies at least one of these allocations. The Fighter class, for example, always requires Strength to be specified as Primary.
When performing an action and where the activity would draw on a Primary characteristic, you may add 6 to the roll.

2) Very high or very low characteristics attract penalties or bonues to any and all d20 rolls.
The penalties/bonuses are as follows:

1 -4
2 to 3 -3
4 to 5 -2
6-8 -1
9 to 12 0
13 to 15 +1
16 to 17 +2
18 to 19 +3

Saving throws

The categories of saving throws are the same in SD&D as in AD&D. However, each category is tied to a specific Characteristic, rather than being independantly generated.

When the character must make a save, the DM specifies (but is not required to reveal to the player) the intensity of the threat, expressed as a number greater than 1 and typically less than 20.

To save against the threat, roll higher than the intensity identified by the DM on a d20.

Bonuses for the characteristic being tested are applied to the roll. That is: if the characteristic is Primary, you may add 6. If the characterteristic is unusually high or low, add or subtract that amount.

The category of save and the characteristic that it is based up on are:

Paralysis, constriction Strength
Arcane magic, illusion Intelligence
Divine magic, confusion, gaze attack, polymorph, petrification Wisdom
Breath weapon, traps Dexterity
Disease, energy drain, poison Constitution
Death attack, charm, fear Charisma
Spells Variable

Performing class-related activities

Every class has a list of activities, or skills, that we allow the character has some expertise in.

Fighters, for example, are skilled at combat, though any fool can give it a go.

In 1st edition AD&D the annoying mechanism of a percentage roll existed to cater for such things as thieving skills (Thief class activities).

In SD&D any such class-listed skill is resolved the same as combat. The DM assigns a level of difficulty and the character must beat it on the roll of a d20.

The advantage that the character who is class-skilled in the task has is the automatic primary attribute bonus of +6 on the roll.

Casting spells

The magic model is simple. If you know how to cast a spell, then only being interupted will prevent you from successfully casting it.

If the spell is directed against an object or entity that is due a saving roll then the effects might be reduced, altered or nullified. But under all circumstances the spell has still been cast.

Having said all that, there is still plenty of opportunity for the DM to interfere to make things interesting…