Brimstone incorporates a very smart and configurable application layer filter. Circuit-level gateways are much like packet filters except that they operate at a different level of the OSI protocol stack. Unlike most packet filters, connections passing through a circuit-level gateway appear to the remote machine as if they originated from the firewall. This is very useful to hide information about protected networks. Brimstone supports both Socks and a manual circuit-level gateway.
Application gateways represent a totally different concept for firewalls.
Instead of a list of simple rules which control which packets or sessions should be allowed through, a program accepts the connection, typically performs strong authentication on the user which often requires one-time passwords, and then often prompts the user for information on what host to connect to. However, for most environments it provides much higher security because unlike the other types of gateways, it can perform strong user authentication to ensure that the person on the other end of the IP connection is really who they say they are. Additionally, once you know who you are talking to, you can perform other types of access checks on a per-user basis such as what times they can connect, what hosts they can connect to, what services they can use, etc. Many people only consider application gateways to be true firewall, because of the lack of user authentication in the other two types. The core Brimstone ACL provides application gateway functionality.