Originally the keyword didn't exist but was simply part of how walls functioned, as walls couldn't attack under the original rules. In Kamigawa Block this was changed and all existing walls were errated to have defender.
Alan controls a Wall of Swords, a 3/5 creature with defender, and wishes to attack. However, because Wall of Swords has defender, he cannot declare it as an attacker.
- 702.3a. Defender is a static ability.
- 702.3b. A creature with defender can't attack.
- 702.3c. Multiple instances of defender on the same creature are redundant.
First Strike • Flash • Flying • Haste • Hexproof • Indestructible
Lifelink • Menace • Prowess • Reach • Trample • Vigilance