The most important and highly fortified component of a Medieval castle was its gateway. It allowed the inhabitants of the keep to come and go while barring entrance to its enemies. Firewall gateways perform the same function.
Firewalls allow access to the outer world while shielding the protected areas from attack.
Permission to access the protected network must rely on secure authentication of an individual. Management must not consider the installation of a firewall as the solution to all of its security problems when connected to the Internet. Firewalls provide a wide variety of control, but in the end they are only a tool. A firewall is part of a diversified defense strategy that identifies what must be protected and identifies the potential threats. It seems obvious, but there is more to protecting a network than hardware and software.