interesting question .. it is the genoese gateway ... there are many theories about it .. My preferite is that it is a symbolic rappresentation of the city of Genoa itself . The medieval name of the city was infact IANUA that in latin means door or gateway. Genoa at the time was probably the most important mediterranean commercial port and the majority of goods destined to the today North Italy , Swiss and South Germany passed through it, so it was a sort of geteway to the central Europe.
This Construction can be found on the first and almost all the genoese coins struck from the 1139 to the 1638 , including also many of the coins struck in his colonies. The gateway became a sign of quality and for that reason it was maintened for a quite long period, but was reproduced on many coins of others country in slightly different ways as for example on the Crusaders states coins.
Here some examples of genoese coins reproducing the gateway