What is the drill camp for?

The drill camp lets you spend all your hero XP on just five heroes. So choose five heroes and level them up evenly as much as possible. They will automatically become the leaders in the drill camp. Put all your low lvl heroes into the camp slots and they will level up to the lowest level of your leading heroes. You can leave your low lvl heroes in there permanently - if you take them out, they will revert to their actual lvl. It’s a way to get ‘bang for your buck’ with hero XP..

Drill camp allows you to raise the level of many heroes without investing. It looks at your top 5 heroes and raises the rank of lower heroes to the lowest level of one of the top 5. Just select the picture you want upgraded.