Wall of Ice/1.03.24

Temples
Walls of Ice are created through the Ice magic spell that shares the same name and requires 8 points of mana to cast. They are slightly wider than the standard wall sections found within each races' walls / towers building list (3x2), but are slightly weaker by 100 hit points and come in one aspect only. Their sole purpose is to be a durable obstruction that can be raised immediately when you have need of them, adding a blockade between the enemy and your ranged units / buildings. Comically, they can be summoned around enemy towers to obstruct their firing path - preventing them from being able to successfully fire off one of their non-electrical ranged projectiles.

Quite early in the campaign, thanks to the Summon Mana research, it is possible to evoke and maintain (renew) half a dozen of these wall sections, sparing a hefty sum of gold and stone until the economy is on track. The duration of the spell is 10 min at mastery level 1, and as noted in the trivia, a Wall of Ice costs virtually nothing to repair (20 Crystal at max)!

Trivia
-Game files indicate that this building costs 20 Crystal and has a build time of 10 despite it not being build-able. That should explain however why it takes so few crystal to have them repaired.