Symbols sound like Cryptology to me. I know very little so far about the game but I have heard that in game mail will have to be physically delivered and could be passed off to couriers. Elyria will be very vast, making information across long distances very valuable. If this is explored it opens up a world of possibilities on the espionage level.
The lack of instantaneous mail delivery means it can and most probably will be interrupted along the way. If there is important information, things like cryptology or even personal character or house seals will be key factors in Kingdom Defense.
Let us say that there are two Kingdoms in a tense peace with each other and they are called KA and KB. Both Kingdoms are direct rivals and would love to trample on their opposition, however their citizens are wholly uninterested in a major war. All they need is a Casus Belli, some justification to goad their citizens into battle.
KB has a Spymaster while KA has Strategists. KA decides to spark the war by goading KB into taking action first. They send key information for this plan to the border between the two nations using an honorable courier. KB has been waiting for this moment however, and an agent steals the mail from an unknown courier. They break the seal and copy the information.
Now, the fun part. They reseal the envelope using a heated knife and return it to the courier unawares to get it delivered. The plan will happen, but KB can create a counter plan now. Except, the real information was actually written with lemon juice on the envelope paper itself! The strategist was aware the mail could be tampered with, and indeed, the seal showed signs of tampering.
Did the spy from KB notice the real hidden message, or did they only catch the red herring clues? Both sides think things are going according to plan, but who will get the public on their side?
Imo, players morale will be very important in supporting counties, duchies, and kingdoms. Considering the distance we will already be travelling, players are more likely to be interested in border skirmishes and monster fights than a prolonged drawn out war. Short battles will hold their attention, but a kings real challenge would be to get them to commit to the long haul with this sort of thing.
Wars are a sort of high risk, long return investment. Too much information will be travelling at a time for anyone to truly control it, and that is just how it should be. Having the information being so important would raise the importance of investments with quicker return such as alliances or trade.
I might say more on this later, but I'm losing my train of thought so I shall stop here on a good note. :)
TL;DR Cryptology sounds amazing. We could also have mail seals, lemon juice, mail theft, and other such fun things.