Historians believe that it was built as an estate for Pachacuti, so they were able to do the sophisticated construction despite not having a written system as we know it. They had a system of knots for some forms of record-keeping, but despite that, they were able to have a sophisticated society. Their way of taxing people was not through formal coinage. They didn't have a monetary system as we know it. Members of their society had to dedicate a proportion of their labor to the empire, to the emperor, and this might seem different than what we do, but if you think about it, if my income is taxed at 30% or 35% in, say, the US, essentially what I'm giving to the government is 35% of the work of my labor. I'm just doing it through a monetary system. They did it directly.