Yes, this is one more event that I like in the game. I took part once a few months ago perhaps.
A little briefing before one enters the Battlefield.
Select a faction. I just select the Eagles.
So I started from the Eagle Roost. The aim is to conquer other factions' camps so you can gather more resources to buy troops and score.
These are the basic troops. I think they all have 40 attack points and all travel at the same speed. The problem is, one can only buy a limited number of troops in accordance to the troop load. In my example only 45000 soldiers. Only after a few attacks did remembered to assign my dashing Jarvis.
These are the resources needed in the Battlefield. Get them by clicking Gather icon every 5 minutes (and I often forget during attacking). Ancient coins can buy you soldiers and crystal relics can be exchanged for the coins. Crystal Relics can also be obtained from attacking camps.
In this example, I have 2929 ancient relics and I can exchange one relic with 50 ancient coins.
How to attack? Aim for the lowest percentage of city protection. The lower the percentage, the fewer attacks you have to launch and save soldier deaths.
Like the above example, I wait for other people to attack that camp first and then I attack it.
Example of a battle report. And it also reports how much relics looted from the attacked camp.