Apparently, during the episode, I think Max and Katie said they hope they were making a smart decision by not digging a sandcastle that they know has been rebuilt by another team. Hence, a Lena and Kristy moment could happen here should a team not DIG deep enough for the clue and then went on to rebuild the sandcastle and then all the other teams virtually will assume that there's no clue in there and not find a clue from it. I think it's kind of easy to tell which one was a sandcastle built by the production and which one was rebuilt by the teams based on the wetness of the sand.
Considering the odds, this task could be way harder than the Haybales task in TAR 6 and 15? Because the odds are decreasing exponentially with each clue found, for example, 3 to 4:400 sandcastles for the final three teams 3:whatever that's not been unrolled for the Haybales challenge in TAR 15 and we know there's only 186 of them. The strength required for both tasks are varying but let's not forget the heat factor, dehydration etc.