A colossus stands in front of a gateway. To its right side, there are three pedestals. Each of them is dedicated to an animal: a bird, a rabbit, and a frog. The player can interact with each of the pedestals, thus increasing a counter in the statue's upper part to values between 1 to 4 and starting from 1 again after 4 has been reached.
With the right combination, the colossus will disappear magically, providing access to the Rocky Hearts.
Currently, the solution to this puzzle is currently only available in Story Mode, but not in Survival/Endless Mode. According to the author, the Rocky Gateway will be removed for the latter in one of the next game versions. It is not hard to simply brute-force the combination; there are only 4^3 = 64 possible combinations, which can all be tried within a few minutes. If you know even one of the correct positions that number decreases to only 16 possible combinations.
Visiting the Rocky Gateway takes a quarter of a day.