One advantage of firewalls, however, is a relatively high return on investment. One or two firewalls are usually just as effective for controlling external access to network services as implementing access control on every single server on your network. And using one or two firewalls will probably be much cheaper, both for initial cost and long-term maintenance.
Firewalls can also be used to give you an idea of how much hostile intention your network is earning and what kinds of attacks are being attempted and hence what you need to concentrate on. If you don't have a firewall, you should make sure you have good backups of all your data, and then contact a firewall vendor.
Kurt Seifried (email@example.com) is a security analyst and the author of the Linux Administrators Security Guide.