When houses reach a luxury quality level, their residents stop contributing to the city workforce. This can lead to labour shortages and industrial stagnation, so the player must be careful to ensure that some houses do not evolve past this threshold, even though the tax benefits are great. In later games, starting with "Zeus", housing classes are separated. As a result, housing of a given class can only advance to the peak of that class's housing quality; to obtain luxury housing and tax-heavy aristocrats, it is instead necessary to establish separate elite housing plots for them to move into. In "Caesar IV", the housings are further divided into two different classes of workers as well as the non-working elites.