A. Use NTP to make sure that time stamps of log messages are synchronized such that you can do root cause analysis.

B. Besides using a dedicated firewall / DMZ construction use an IOS based firewall on the WAN router connecting the industrial site to the Internet.

C. Deploy an IDS solution with knowledge about industrial protocols in the industrial zone in combination with a firewall.

D. Use multiple firewalls from different vendors in such a way that network traffic will have to traverse both firewalls so that security holes of one firewall is caught by the other firewall.