Defense in Depth. As a philosophy it means “Don’t put all your eggs in one basket”. Assume that each layer of your system can be (or has been) breached, keep slowing the attacker. Be less attractive than others who might be attacked.
I presented on this topic with respect to Cloud Native (and various CNCF tools) at Cloud Native Day Montreal. See the presentation online below (or linked here). In it I go through some common myths, things to watch out for, and things that we are doing that are lazy and dangerous.