For a literal lion's dwelling:
* Lair: This is the most common synonym for a lion's den, meaning a wild animal's home.
* Cave: If the lion's den is in a cave, this is a suitable word.
* Burrow: If the lion's den is a burrow, this word is more specific.
For a metaphorical lion's den:
* Fortress: This suggests a place of strength and protection.
* Sanctuary: This implies a place of refuge and safety.
* Hideout: This implies a secret place for hiding or planning.
* Headquarters: This implies a place of command and control.
* Base: This implies a place of operation.
The best word to use will depend on the specific context and the meaning you want to convey.