Pyrimidine (pym) ligands with their two endocyclic N‐donor atoms provide 120° angles for molecular constructs, which, with the 90° angle metal fragments cis‐a2MII (M=Pt, Pd; a=NH3 or a2=diamine), form cyclic complexes known as metallacalix[n]arenes (with n=3, 4, 6, 8, …︁). The number of possible isomers of these species depends on the symmetry of the pym ligand. Although highly symmetrical (C2v) pym ligands form a single linkage isomer for any n and can adopt different conformations (e.g., cone, partial cone, 1,3‐alternate, and 1,2‐alternate in the case of n=4), low‐symmetry pym ligands (Cs) can produce a higher number of linkage isomers (e.g., four in the case of n=4) and a large number of different conformers. In the absence of any self‐sorting bias, the number of possible species derived from a self‐assembly process between cis‐a2MII and a Cs‐symmetrical pym ligand can thus be very high. By using the Cs‐symmetric pym nucleobase cytosine, we have demonstrated that the number of feasible isomers for n=4 can be reduced to one by applying preformed building blocks such as cis‐[a2M(cytosine‐N3)2]n+ or cis‐[a2M(cytosinate‐N1)2] (for the latter, see the accompanying paper: A. Khutia, P. J. Sanz Miguel, B. Lippert, Chem. Eur. J.2011, 17, DOI: 10.1002/chem.2010002723) and treating them with additional cis‐a2MII. Moreover, intramolecular hydrogen‐bonding interactions between the O2 and N4H2 sites of the cytosine ligands reduce the number of possible rotamers to one. This approach of the “directed” assembly of a defined metallacalix[4]arene is demonstrated.