A circuit-level gateway is a type of firewall.
They monitor TCP handshaking between packets to determine whether a requested session is legitimate.
Firewall traffic is cleaned based on particular session rules and may be controlled to acknowledged computers only.
Circuit-level firewalls conceal the details of the protected network from the external traffic, which is helpful for interdicting access to impostors.
Circuit-level gateways are relatively inexpensive and have the advantage of hiding information about the private network they protect.