An integrated circuit 78 is formed of multiple layers of circuits 14, 16 superimposed to produce stacks of circuit blocks 2, 4. Stack control circuitry 18, 20 is associated with the input and output signals from the circuit blocks to direct these to/from the currently active circuit block(s) as appropriate. The superimposed circuit blocks 2, 4 provide redundancy for each other, both for manufacturing defect resistance and for operational redundancy, such as providing multiple modular redundancy in safety critical environments.