Players who do not discuss diplomatic treaties with their neigbours are doomed, and will soon be conquered by an alliance of more than one player. Most talk happens on our Discord server, and there are some alternatives if you don't like it.
The forum is used for announcements that should remain available for a while. Game-specific rules are posted there, as well as victory declarations. It is common practice to cross-post everything on Discord. The forum can be accessed at forum.longturn.net.
During the game, you can use the in-game chat to talk to other players. The game chat supports links to tiles, cities and units, which is very useful to coordinate an attack with your ally. You can also use colors etc; see the Help/Chatline menu in your client to learn the full feature set.
IRC is a way to contact the admins should everything else fail, provided they are online. Join #longturn on irc.freenode.net.