ajb... Well, You "Lucked Out" with having an easy correction for your Biq. This is a Lesson in SAVE Backups just in case
The only way I know to absolutely prevent an Improvement Building from being destroyed is if it is placed by a Great Wonder or if it is flagged to allow Level 2 or 3 Population increase. Of course if and when the Great Wonder is Captured or the City that has it is Razed, the Placed Building will be gone. That said, you could make the Building a Great Wonder. Then it could not be Bombarded away. If you want the Building in a Particular City, set a prerequisite in the City you want it in such as another Wonder, Improvement or set Resources required within City Radius.
Alternately, you could set the Building to build at a very low cost so when it is destroyed, it can be rebuilt immediately.
The Characteristics Flags affect more than a "Golden Age" but No Flagged Characteristics will prevent the Improvement from being destroyed from Bombardment other than what I mentioned above.
The only way I know to absolutely prevent an Improvement Building from being destroyed is if it is placed by a Great Wonder or if it is flagged to allow Level 2 or 3 Population increase. Of course if and when the Great Wonder is Captured or the City that has it is Razed, the Placed Building will be gone. That said, you could make the Building a Great Wonder. Then it could not be Bombarded away. If you want the Building in a Particular City, set a prerequisite in the City you want it in such as another Wonder, Improvement or set Resources required within City Radius.
Alternately, you could set the Building to build at a very low cost so when it is destroyed, it can be rebuilt immediately.
The Characteristics Flags affect more than a "Golden Age" but No Flagged Characteristics will prevent the Improvement from being destroyed from Bombardment other than what I mentioned above.