Hi there! It's an awesome game all in all, I finished the whole prestige tree so I pretty much finished the game.
One thing that the others haven't mentioned is that in super late game (for instance I'm at level 68), it's almost impossible to get mithril because the HP of the dens is even higher than the HP of the wall itself. I think this is a game design error because it's almost impossible to destroy the dens from a certain level upwards which in turn makes it very hard to get mithril consistently.
That being said, I had to resort to focusing all my upgrades on a single type of unit otherwise the spread of mithril makes it unreliable to fully upgrade everything.
And, like others said, the runners cannot effectively pick the gems even if they're protected: I have 10 lasers that destroy rocks, I have about 10 turrets maxed out, and the runners themselves are maxed out. Even so, the calamities are triggering so fast that there's barely any runner coming back with a full load.
What happens is that the calamities themselves are pushing the rocks towards the base and with the amount of gems I have lying around, it's enough so I can get money in but the runners don't appear to be useful enough.
Again, all in all this is a GREAT game and I hope more people get to enjoy it. I'm really looking forward to the final version and I hope you find this feedback useful :)
All the best! 
