Defense in Depth/Layered Security is more likely a Core Security Goal for a company, it's a concept or should I say a practice of implementing several layers of protection. You can't simply take or risk a single action by just implementing a firewall or an anti-malware software, and consider yourself protected. You must implement security at several different layers, in this way, if one layer fails, you still have several layers to protect you. It's common now a days that a company implements intrusion detection system (IDS) or intrusion prevention system (IPS), Firewalls, proxy servers for content filtering, anti-malware for every works station, HIDS/HIPS for critical servers or static environments.
Network Security Policy is more likely a generic document that states appropriate expectations regarding the use of corporate IT assets. This is closely connected to Acceptable Use Policy (AUP) but this one is focusing on workstations. In this document, organizations usually state their objective or core goals in protecting assets and the risks of not following these policies.