EDIT: For clarity, the Forts appear after you complete Vision of Ice and Vision of Fire, Tensay's trippy missions.
Important to understand that 'Forts' are not the same as outposts or campfires. In fact, there are only two Forts in the game. Once the two rival tribes are established their leader's icons will appear on the map. A short distance from both will be a smaller icon - this is the base of their respective commanders.
Both missions are rated very hard. I took both on fairly early skills-wise - I focused skills on my owl bee drops and cooldowns and took out all but the leaders with a combination of the owl and sending in a cave bear after the tougher enemies. Reinforcements will be called this way but depending on how well you have camped you can avoid most of the fight. Both forts are fairly out in the open with good hiding spots nearby within the owl's firing range.
However for the commanders themselves you will likely have to get in closer to do any real damage - they actually have HP bars that you must whittle down. As you might expect, the Izila uses fire and Udam uses gas, so consider tooling up your health recipes before attempting.
It's a balance - the earlier you go for the Forts the earlier you will have access to additional bombs (fire bombs from Izila fort helped massively against the Udam fort, for example). But going early will mean taking it slow and guzzling up resources to keep your bear alive and your owl stocked up.