One idea I had was to modify how Fortification works.
Current implementation:
If a unit is allowed Defensive Bonuses, then a check is done at the start of the turn to see if it moved last turn. If it did not move, then the fortification level of the unit is increased 1 rank, to a max of 5. If you have the Defensive Promotion, it is increased 2 levels each turn, still to a max of 5.
If you ever move, the instant that you do so your fortification bonus is removed.
Idea for a new approach:
Fortification Level is still bumped up 1 rank if a unit did not move the last turn, and this happens to a max of 5. But if the unit's fortification level is OVER 5, then it is reduced by (Fortification Level / 5) points (so a unit between 6 and 9 loses 1 Level, between 10 and 14 the unit loses 2 levels...). This means that for a standard unit, they can get normal results from fortification. For a unit with Defensive, they will fortify to 10/20/30/40/50/55/60/65/75/85/95/100 (stepping up 2 levels per turn till 50, then 1 level per turn beyond that)
Now... to make use of this change, you need to have a method of increasing your fortification bonus beyond the normal level. To do that, you modify Treetop Defense to increase the fortification level of all units by 5 (instead of just capping it instantly, since now there is no cap) and it applies a promotion which grants 5 levels of fortification per turn (to prevent the need of casting the spell each turn to reach phenomenal fortification caps).
Naturally, Earth Elementals would gain a spell which they can only cast if they have not moved or attacked for the turn which grants them a massive fortification bonus, as well as pumping up their chances to defend the stack.
And finally, the reason that I am posting the idea here, is that the Dural would gain natural access to either a spell or promotion which enhances their fortification capabilities. My thoughts initially were a in-City only promotion which grants a large fortification bonus, but it would also be nice to give them a 1-shot spell which can be used in enemy territory when forced to go on the offensive (spell requires you haven't attacked for the turn and have a fortification bonus lower than 80% and it sets your fortification bonus to 100 instantly... something like that)