Wooden castles were converted to stone castles as—

The correct option is stone castles offer better defense.

Step-by-Step Explanation:

1. Historical Context: Early medieval castles were often made of wood (such as Motte-and-Bailey castles) because timber was readily available and quick to assemble.

2. Vulnerabilities of Wooden Castles: Wooden structures were highly vulnerable to fire attack (arrows dipped in pitch, incendiary projectiles) and could be easily damaged or destroyed using battering rams, axes, or siege engines.

3. Advantages of Stone Castles: Rebuilding or upgrading castles with stone provided significantly improved defensive capabilities:
- Fire Resistance: Stone does not burn, rendering fire attacks ineffective.
- Structural Strength: Stone walls were far thicker and stronger, able to withstand heavy bombardment from siege engines like trebuchets and mangonels.
- Durability: Stone structures offered long-term stability and could support taller towers, giving defenders a strategic height advantage and a better viewpoint.

4. Conclusion: Therefore, the transition from wooden castles to stone castles occurred primarily because stone castles offered far superior defense against attacks.