The Rusty Key is good for all upgrades for the Mausoleum

These Rusty Fragments can usually be gained after unlocking a new enemy. Like brains and other items, Little Empire Mojo   Rusty Fragments that make up the Zombie Farm Mausoleum key can be gained as loots from invasions. Little Empire Mojo  You have less chances of getting them with Old McDonnell, but you’ll have a higher chance of getting them once you’re at level 26.  Buy Little Empire Mojo  Once you’ve obtained the Rusty Key, you’ll gain access of the Zombie Farm mausoleum and initially store 9 zombies. The Rusty Key is good for all upgrades for the Mausoleum. You no longer need to obtain another set of Rusty Fragments to unlock the upgraded versions of the Mausoleum. The zombies inside the Mausoleum can also be taken to invasions. Cheap Little Empire Mojo  They also get hungry even if they are inside the Mausoleum so you can still take undistracted zombies to battle even if they came from the Mausoleum.