General:
* Sturdy: This is a common word for something strong and well-built.
* Solid: This emphasizes the feeling of being dense and substantial.
* Robust: This implies strength and resilience.
* Durable: This highlights the ability to withstand wear and tear.
* Substantial: This suggests a feeling of weight and importance.
More specific:
* Fortified: This suggests that something is built to withstand attacks or damage.
* Impregnable: This implies that something is practically impossible to break into or destroy.
* Well-constructed: This emphasizes the quality of the workmanship.
* Massive: This describes something that is large and heavy.
* Immovable: This suggests that something is fixed in place and difficult to move.
Figurative:
* Rock-solid: This is a metaphor for something that is extremely strong and reliable.
* Unbreakable: This implies that something is impossible to break.
The best word to use will depend on the specific context and the feeling you want to convey.