Here is an interesting essay by Otto von Habsburg. An excerpt;
" Republicans frequently claim, in addition, that monarchy is a form of government belonging to the past, while republicanism is that of the future. Even a slight knowledge of history is enough to disprove this. Both forms have been in existence since the earliest times (though the monarchical periods have usually lasted considerably longer than the republican ones). In any case, it is misleading to call an institution which we already find in ancient Greece, Rome and Carthage, the form of government of the future."