Question: How to make sure files never get destroyed.

Let's say I run a server and on this server I have several users to which I have given access to several folders which hold a lot of data, say 1TB. This is required for them to do their jobs. They need to be able to make, edit, and delete files. But, I want them to be backed up often and I want to make sure that the users can not delete or edit the back ups. I also want to make sure that if the building in which the server sits gets destroyed the backup would be safe. How would you suggest I do this? What permissions/groups do I use? What software do I use to back these folders up? How often can I expect to be able to back stuff up? Have I missed anything super important? Thank you.