Multiplayer will allow you to play with others. You can browse or start your own host.
The host can create a private game by clicking the Private check mark within the lobby. This hides the game from the "browse" option on the multiplayer menu.
Players can now join the game directly by choosing join and specifying the IP.
Make sure you don't have hammerwatch blocked by any firewalls and open port 9995 both udp and tcp.
To host a game you may need to port forward 9995 udp and tcp. This is mainly an issue if you have router responsible for connecting your internal network to the internets.
Many routers have uPnP enabled by default so port forwarding may happen automatically. Please consult your router documention on uPnP before enableing it to fully understand the scope of this option.