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: 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, Sun, Magma, Smoke, Light
Water, Acid, Ice, Steam, Blood
Air/Wind, Lighting, Storm, Fog, Sound
Earth, Metal, Wood, Sand, Gravity
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.
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>