Hero shards are great because they allow you to recruit heroes. However, there are not that easy to get and requires intensive farming to get it in Castle Clash. If you are unwilling to spend money, hero shards and honor badges are the two ways in order for a free to play user to get heroes.
Hero shards has a small chance to drop in every dungeon, but there is a higher chance to drop in dungeons that have a skull marked on it. Those dungeons are also ideal for training your heroes.
It takes a lot of shards to get a hero, but it is worth it as you are eventually able to get a legendary hero.
New Method! – Monster Siege
With the new patch update, you can now do Monster Siege, located at the top left of your map. Clicking on the portal allows you to enter Monster Siege. Waves of monsters and heroes are going to infiltrate your base just like an opponent would. If you successfully defend your base, you will get hero shards(up to 60 per siege). You can do them once every 3 hours and you have a max entrance count at 4.