Domain controller placement in the infrastructure is dependent on a few things:

• Network topology: Organizations can have different buildings, branch offices, and data centers connected together. Services and resources will be hosted in those locations that will require domain controller integration. Replication is key for domain controllers. The placement of the domain controllers in the network will depend on whether it's possible to achieve successful replication or not. Network segmentations can prevent relevant traffic from passing through networks in order to have successful replications. It is important to adjust the network topology to support the Active Directory design you have in place.
• Security: Physical security is important for domain controllers as its holds the identity infrastructure footprint. In places that you cannot guarantee physical security in your network, it is recommended that you do not place the domain controller. In such scenarios, instead of the domain controller, it is possible to deploy a RODC.
• Link reliability between sites: As I mentioned earlier, replication is key for the health domain controller infrastructure. If the connectivity between sites is not stable, it is not possible to place the domain controller and maintain healthy replication. In such scenarios, it's advisable that you use RODC.
• Active Directory sites: We covered Active Directory sites in Chapter 1, Active Directory Fundamentals. It is important in the physical topology design. In the later chapters, I will demonstrate how to set up site links and how to manage them.