Once the harvest and trading finish.... -> Once the harvest and trading finished.....
The construction was progressed rapidly with the minotaurs helping --> The construction was progressing rapidly with the minotuars helping.
and that unboxed text between the running centaur is not the same as the box text. Its more like "in the open, they're pretty strong and fast".
also there was a double space between "minotaurs" and "helping".
there're -> just no. Please use "there are" instead. You have room for it.
Do you use them as ride --> Do you ride them? Or Do you use them for riding?
@Chizan
Probability-wise because they take shifts and given their number it is unlikely that he "does the deed" with any particular one more often than once per month, thus (given the medical knowledge in this world and his lacking in that particularity) it is also far less likely to hit 'em up on the right day of the cycle, so take that into account as well.