[@The Harbinger of Ferocity] Just the kind of questions I need to get my thoughts in order.

Let's start with player roles.  For heirs to the kingdom I was planning on perhaps two, three at most.  As for other roles people could choose, they could be other members of the royal court that escaped, guards, champions, mages, etc.  Players could also be members of the nation the heirs had to flee too, those who wish to help for various reasons or sent there by the nation to act as spies and advisers to make sure events transpire the way they want to.  Most would prefer the heirs to have some degree of success to make sure the invading force don't get any funny ideas and to put the heirs in their debt.

As for tropes like magic let's see.  While important I'd rather it not become too center stage and be able to instantly change the course of battle with one mage.  The way magic will work is that each person will have a rather limited mana pool, meaning if you want to be effective with magic you need to learn to be very efficient as an individual, or you need to pool a bunch of people's resources together for more devastating effects.  As magic itself is rather limited in amount, many 'mages' would probably have knowledge of alchemy and pharmacy to lean back on.  Once mana is used up it will take some time for a mage to recharge their supply, be it through natural sleep or meditation.

As for the types of magic there will be three main types: The Elements, Blessings, and Curses.  The Elements are the manipulation of well, the elements, Fire, Air, Water, Earth, etc.  Because this is a manipulation of nature this tends to be the most draining magic to use and can be used offensively the most easily, such as conjuring a fire blast.  Blessings in general are ways to invigorate the body to help soldiers fight better or to increases a body's natural healing processes.  You can think of Blessings as mostly just a small buff, won't make anyone superhuman but could be the edge needed to win a close battle.  Finally there are Curses, mostly only useful in a one on one fight and tend to be more mental than physical.  They can have various effects such as making someone more sensitive to pain, making them more likely to freak out, adding a little brain fog, etc.

As for fantasy elements like monsters, they do exist but are very few in number and in general can only be found in uninhabited areas.  Mankind has done a pretty good job in exterminating any near human civilization, as such while there may be a few monster hunter specialists there is nothing like an Adventurer's Guild designed just for monster extermination.  Basically there is a very low chance of running across a monster in the wild unless you were looking for them in the first place.