Myth II, starší hra. VE FAQ toho serveru (mariusnet.com) se píše tohle:

When people try to join my host they get a firewall error, how do I fix that?
This error means that there is a piece of hardware or software preventing people from connecting to your host. If you are behind a router, or Network Address Translation (NAT) firewall, you will need to forward the port 3453 to the computer you intend to host with. This website (http://portforward.com/routers.htm) describes the process in detail for most consumer routers. It is also possible you have a software firewall preventing incoming connections to your Myth host. Ensure that any software firewalls (like windows firewall, comodo, zonealarm, etc) have been properly configured to allow myth to accept incoming connections.