A three-dimensionally structured gate resembling a pillared hall allows the exertion of a strong potential modulation on a two-dimensional electron gas in a GaAs/AlGaAs heterostructure. The system can be tuned over a wide range, from a metallic, open antidot lattice to an insulating system of disconnected dots, by a negative gate voltage. At threshold each dot typically contains 50 electrons. The conductance near threshold shows strongly activated behaviour at liquid helium temperatures, which can be modelled by considering tunnelling through a barrier between two dots.