The concept of security in depth is recommended in this environment , which is perforemed by using different security mechanisms Like firewall, DMZ, detection system, security polices, encryption and etc.
furthermore, network segmentation and segregation into different enclaves which are controled by a set of rules for the communications which are permitted to pass or not( and this is done using also firewall), but here I mean the idea of enclaves which can be logic or physical, make this environment more secure and manageable.
so, for this hetergenous environment multi-tier , and overlapping security machnisems is more adequate.