Monolithic 3D ICs have vertical interconnects that are comparable in size to local vias, thereby permitting extremely fine-grained vertical integration. SIMON, a lightweight block cipher, is designed and characterized at the Graphic Database System (GDS) level in two types of monolithic 3D design styles: transistor-level, where nMOS and pMOS transistors are split between tiers, and gate-level, where individual gates are partitioned among the tiers. The two 3D implementations as well as a 2D implementation are compared and characterized in terms of area and power. Furthermore, the effect of monolithic intertier vias (MIVs) on power and data integrity is analyzed for each custom 3D design. It is shown that power delivery for transistor-level monolithic 3D design is more challenging since all of the pMOS transistors (that are connected to the supply voltage) are located in the bottom tier where there are limited metal resources due to technology constraints.