As you can see the picture above, the highest reachable Heroes' level is 45 (through gaining experience or buying experience with gold). However, you are able to somehow perform "Level Unlock" with 45 blue Gems as stated. Technically speaking, it is almost impossible for me to unlock the next level without paying. The cost of 45 blue Gems is 1,000,000 Golds (it would take me hours or days to obtain such a huge amount through unlocking stages). Moreover, one thing to point out is that, in total i have only unlocked 9 Heroes out of 24. I really hope that the maximum level of 45 is enough for me to go through all the stages left, otherwise i would just stop playing this game. :p