  The Cherenkov Telescopes Array (CTA) is planned as the future instrument for
very-high-energy (VHE) gamma-ray astronomy with a wide energy range of four
orders of magnitude and an improvement in sensitivity compared to current
instruments of about an order of magnitude. Monte Carlo simulations are a
crucial tool in the design of CTA. The ultimate goal of these simulations is to
find the most cost-effective solution for given physics goals and thus
sensitivity goals or to find, for a given cost, the solution best suited for
different types of targets with CTA. Apart from uncertain component cost
estimates, the main problem in this procedure is the dependence on a huge
number of configuration parameters, both in specifications of individual
telescope types and in the array layout. This is addressed by simulation of a
huge array intended as a superset of many different realistic array layouts,
and also by simulation of array subsets for different telescope parameters.
Different analysis methods -- in use with current installations and extended
(or developed specifically) for CTA -- are applied to the simulated data sets
for deriving the expected sensitivity of CTA. In this paper we describe the
current status of this iterative approach to optimize the CTA design and
layout.
