A Compound of 13 Cubes
I don't remember when I built this one exactly, but it was some time during the 90s. The design for this model is easy. I just took the compound of 12 cubes as shown here and added one cube according to the symmetry. Easier said or thought than done. At that time I had to calculate pieces algebraically by hand. And building the models takes time too. All that took several months alltogether (I don't remember how much exactly). Anyway I was very satisfied with the result. Even though the model isn't that special.
Here is a picture more or less taken into an order 4 symmetry axis: