A Combinatory Automaton is a simple tool to simulate combinatory systems. This is composed of a lattice, each of whose cells contains a variable representing the state of an agent. The value of each cell at time th depends on a synthetic global variable whose values derive from some operation carried out on the values of the cells; this variable represents the synthetic state of the automaton. The micro-macro feedback connects the analytical values of the cells and the synthetic state of the automaton. With the aid of a simple probabilistic combinatory automaton I shall simulate a very strange but meaningful phenomenon: the formation of the towered town of Pavia, Italy, which is unique in all the world, as can be observed in the historical descriptions or by a visit to this ancient Italian town.