要塞

1. Origins, structure, and basic meaning

• Structure: 要塞(ようさい) means a fortified military site or defensive stronghold built to resist attack.
• Basic usage: It is used for a fortress remaining on a mountain or for turning a location into a defensive position.

2. Common pitfalls and similar expressions

• What changes and what it becomes
  • 谷の入口を要塞に変えた means turning the valley entrance into a fortress.
  • を marks what is changed, while に marks its resulting form or use.