Lightning Lock

A Lightning Lock is a magical barrier that will electrify anyone who attempts to pass through.

A Link to the Past
The Lightning Lock is placed by Agahnim on the Hyrule Castle Tower, blocking anyone from entering. Link can witness the Lightning Lock early in the game but cannot destroy it. As it is created by Agahnim's evil magic, it cannot be destroyed with anything less than the Master Sword. Striking it will simply cause Link to take damage.

Ancient Stone Tablets
In the Lighting Lock is placed on Ganon's Tower, blocking the hero from entering and challenging Ganon for the final battle. Even with the Level 4 Sword he cannot destroy this barrier. The hero will have to travel to the Lost Woods to obtain the Master Sword if he wishes to break through and enter the tower.

Trivia

 * Similar electrified barriers appear in and, also at Hyrule Castle in each game. In The Wind Waker the barriers also appear in Hyrule Castle, and can only be removed by destroying all the enemies in the area. In Four Swords Adventures, only the Fairy Queen can remove the barrier blocking the way to the tower of Hyrule Castle.