  The Cherenkov Telescope Array (CTA) will be the next generation
very-high-energy gamma-ray observatory. CTA is expected to provide substantial
improvement in accuracy and sensitivity with respect to existing instruments
thanks to a tenfold increase in the number of telescopes and their
state-of-the-art design. Detailed Monte Carlo simulations are used to further
optimise the number of telescopes and the array layout, and to estimate the
observatory performance using updated models of the selected telescope designs.
These studies are presented in this contribution for the two CTA stations
located on the island of La Palma (Spain) and near Paranal (Chile) and for
different operation and observation conditions.
