Shard Technology Uses For City-Defenses [closed]

Question

In my Fantasy world there is a magical resource called shards that is as follows:

Shard Powers: For all the people of the world; the benefits and dangers of using shards are well known. Shards are the ultimate capacitors; being able to take in almost any form of energy and store it for use later is a feat unseen in any other materials. Plus, their ability to be changed into multiple elements to be a specialized Shard is truly remarkable; albeit rare. The only downside to Shards are their volatility; and that once broken or damaged; they ooze a toxic gas that deforms, mutates, and kills all foolish enough to be near it. Shards are used everywhere and in everything. The Larger the Shard the more powerful the results. Although shards are quite powerful they do have a limit to as how much they can handle.

These Shards are limited to 5 Branches with there own unique addition elements to these Elements:

(If you need me to further expand on the elements please let me know, I can give more of a more general of idea of what the element can do)

Energy Branch

Energy: It should be noted all shards start as energy before being converted, all other shards cannot be found naturally and have to be created.

Fire Branch:

Fire, Sun, Magma, Smoke, Light

Water Branch

Water, Acid, Ice, Steam, Blood

Air/Wind Branch

Air/Wind, Lighting, Storm, Fog, Sound

Earth Branch

Earth, Metal, Wood, Sand, Gravity

World Setting:

Like I stated at the top Shards are in everything and anything. They essential are used anywhere to provide light to a room, to powering an engine and can even be used beyond that given enough creativity. I would put the worlds current tech level at that being to a WWII with Magic and fantasy elements mixed in.

Question:

Knowing that these elements can be contain within a shard and can be harnessed for power, what are some of the most ideal ways to use these elements for defending a city?

Edit: If there are ways to make this question sound better please let me know I wish to get better>

Answer 1

You mentioned WW2-era tech, so...
Against infantry
"The only downside to Shards are their volatility; and that once broken or damaged; they ooze a toxic gas that deforms, mutates, and kills all foolish enough to be near it."
That's not a downside. Load the shards into a canister similar to a rocket launcher, then fire it over the city walls into the middle of the enemy infantry. I assume gas masks won't be much help.
Against armor
Use a shard launcher again, but this time, load an Air shard. Prime it so that when it hits a tank, it discharges electricity. The metal will conduct the charge and zap everyone inside.
Against aircraft
Air shard again, summon up a heavy storm around the city. Good luck flying in that! The downside of this is that your own planes, if any, can't fly, but you can at least level the playing field in terms of air superiority.
Against ships
Fire shard--concentrate the light so as to burn a hole in the side of the ships, like the myth about Archimedes, only this time it's real; aim for the munitions storage if possible. Alternatively, you can just reuse the storm from the anti-aircraft Air shard.
Against artillery
This is probably the trickiest one to defend against. An Earth shard might work--manipulate gravity so that the shells fall short. You could try combining that with a Water shard to create a steam screen, so they can't target specific locations. Edit: Changed smoke screen to steam screen.

Answer 2

Assuming you could combine these elemental shards to make more complex machines, you could create railguns. You use air shards for electricity to power up your railgun and earth shards for projectiles like tungsten with ferromagnetic rings. Because of the high velocity of their projectiles, you can hit any kind of vehicle (planes, boats or land vehicles) from really far away.

The main problems for a railgun are rate of fire and energy consumption. You could make smaller railguns with permanent magnets instead of charged rails to remove the need of air shards and potentially increase the rate of fire. This would however reduce power and range, which would complement the bigger railguns.

You could also use lasers which would only use air shards (for gas and electricity). They would have very good range but would require a lot of power and continuous exposure. They could potentially be deflected easily as well though.

You could use shards themselves as land mines and for area denial, but winds might not be in your favour and push the toxic fumes towards the city you're trying to protect.

Speaking of which, you could create elemental mines using different shards.

For basic guns, this would probably use air and earth shards (propellant and projectile)

A big problem with all of these weapons is damage done to them as damaged shards would kill or seriously injure the user and people around it.