When castle checkpoints were introduced, the idea was that it would help players to complete floors and get the one-time rewards, especially because it can be extremely frustrating to have to start from the first floor.
I understand it is frustrating when a few bad perk rolls in a run ends your run, but that actually means that your characters are not strong enough to succeed consistently - so although the drop rates are lower at lower floors (at least, for equipment and shards, not sure about hero medals), it’s still useful to get them and therefore I recommend to restart from the beginning than resuming at checkpoints at 10 and 20. Also, they don’t cost much energy (3 energy for each of the first 9 floors, 4 for the next 10) and even that you can recover (or even sometimes have a net gain) from hidden treasures.
However, I do recommend the checkpoint at 30 if
- You are trying to get the one-time clear rewards and feel that you are strong enough to go the distance with 3 heroes at 30 (whereas you might lose one to a bad run if you restarted from 1)
- You have cleared the castle and have a large number of keys for it in your inventory - if you find yourself running out of keys faster than you can replenish them, go back to restarting at the first floor (or, if there’s another castle you can speed farm and shards of the current element are not a priority, switch to it)
- You don’t care about the castle loot and just want to get as many Dark castle keys as possible