The vault was designed by Sir Brenzen as a large, square room to house the civilians in case of an attack. Large amounts of resources are stored within, though very small quantities compared to the current contents of the fortress. While at the time the Vault could have housed all of the fortress's civilians for over a year, the current supplies in the vault are limited and would not be able to sustain the population for more than a season or two.
When the spider-fort was constructed, it was connected up to the vault from the other side. This serves as an alternate route of entry to the vault, but otherwise did not affect the overall nature of the vault.
The contents of the vault at its creation included large quantities of prepared foods and drinks, as well as stocks of wood and cloth. It also contained a pickaxe and a battleaxe, for mining and chopping wood.