It seems that those who are championing the freedom for humanity in EU and and USA cannot seems to champion for freedom of economy. How can you have freedom for humanity if they cannot openly trade with each others without unnecessary restrictions?
It seems that what EU (federation of participating countries) and USA (federation of participating states) practices of major trade blocs are not unlike the old empires, but the age of empires namely Imperial China, Byzantine, Mughal and Ottoman probably fare much better with their openness to trades.
The trade blocs as we know today was started by the Hanseatic League of the Holy Roman Empire that is neither Holy nor Roman nor an Empire [1].
Back in the good old empire days, openness to trades meant letting tradespeople in at all. Others just forbade traders to cross the border, in a closed way like nowadays maybe North Korea. The "openness" consisted of accepting traders, but with tariffs and restrictions on tradeable goods as well as necessary bribes and taxes. Not to mention the nonexistence of any kind of protection along the routes, you just had protection within the walls of a city, and then only when paying the necessary taxes. If you wanted protection for the route, you had to hire your own soldiers.
The granularity of it all varied, usually trade in Central Europe was regulated differently by each city, so you had to pay your dues when crossing the city gates. Others had larger territories, there you had to pay at the border crossings in addition to each city. Oh, and not trading in a city because of taxes? Big no-no, only cities had the right of market, meaning you got in big trouble for that.
I would suggest even the most restrictive trade policy today is more free than in the empires you enumerated. There is only a difference in sophistication of the rules, because nowadays there are far more varied goods and that "unnecessary" stuff like consumer protection.
Your views are very simplistic with regard to how the empires operates. I think that your views are mainly from the European perspectives when at the times they have what you called "dark ages" where the quality of life during the time span period is worst than the time before and after that.
For the rest of the world's empires the arrangement are more sophisticated than that namely Imperial China, Byzantine, Mughal and Ottoman. Most of these empires works like the current trade blocs, and the members of the empires pay taxes to their master [1][2]. Those smaller autonomous states or countries who are not directly under the empires most of the times pay tributary "protection taxes" or Arabic "jizya" where they becomes vassal countries/states of the bigger empires [3]. One of common examples are the Dubrovnik in Croatia which was under Ottoman protection for about 500 years! [4]. At its peak it rivals The City of Venice and this is only one famous examples of a major city not directly under a particular empire but flourished under the protection. I can provide more examples but I hope you get the points.
This arrangement is something like countries not belongs USA but in which under NAFTA trade blocs. This thriving ancient port cities can be like Taiwan today and Taiwan is probably probably moving towards this exact direction [5].
Interestingly since Holy Roman Empire (HRE) is not technically an empire, Hanseatic League of HRE based countries initiated the trade blocs among themselves to emulate a trading under one unified empire. As I mentioned previously this is the basis of our current trade blocs.
You also claimed the non-existence of any kind of protection along the trade routes. This is not particularly true since the Imperial China has extensive guard posts along the the Silk Routes and the distance between them is reportedly one "marhalah" or around 45 km meaning that a horse can run without stopping between these two guard posts. If you like this is the earlier version of the Pony Express since they also used the posts as messengers to deliver the important news (e.g. emperor has died) to the Generals/officials far away from the center of the empire. The Islamic empires also have this similar arrangement and they called this temporary resting places for travellers/traders as "Waqaf". After time some of these Waqafs became large cities [6]. Obviously this arrangement is not security foolproof (similar to today) and they introduced and made popular the concept of "sakk" or "cheque" (meaning contract document) for deferred payment to avoid carrying gold/cash for the traders along the Silk Routes [7].
How the empire is organized internally is beside the point for the trade policy mostly. And yes, my comment is a little euro-centric in that central asia had some states and countries along the silk-road where not every stop along the trade route was a city that wanted to tax travellers. But: all the -as you said- autonomous countries did want their taxes all the same, no matter if they had to pay the next higher organisational layer or not.
You are contradicting yourself (and historic evidence) in claiming that the empires you named are "unified" in any way. The farther (as in, a few days ride) away from the capital you get, the less unified they get, provinces are governed by king-like governors with quite some autonomy, farther out vasall states had to pay tributes and submit to imperial authority, but were autonomous. The Chinese even made it into a world model and philosophy. Regarding trade, you did not point out any imperial mandate unifying that. There was only enlightened self-interest, because trade lined all pockets, so most generally allowed trade. But all the provinces, states and vasalls also had their own policies, taxes and regulations.
As for the protection between cities, there is also a very nice example in one of your named empires: Read up about the Thugee threat before the British got rid of it. There was no protection outside cities, period. Otherwise, even your example about cheques would have been unneccessary.
Nothing you said changes anything about my point, just reinforces it: The empires of old were more restrictive and less unified trade-wise than almost all current nations and trade blocks.
I totally disagree because in some aspects the empires were more unified, open and liberal than today.
For examples nowadays can you bring physically huge amount of cash or gold across borders without being detained? In the empire of old you really can. One of the richest men ever existed in recorded history Mansa Musa, 14th century King of Mali has been reported to travel with tons of golds on the way to pilgrimage in Mecca because apparently his empire held half of world's gold resources at the time [1]. According to historians he so lavishly did he hand out gold in Cairo Egypt that his three months stay caused the price of gold to plummet in the region for 10 years, wrecking the economy for many years [2]. This is probably the main reason modern countries do not allow bringing along your stack of cash or gold in very large amount when you are travelling across borders.
Regarding your not-so-unified under the empire arguments, the city of Dubrovnik in Croatia will not be successful for 500 years (being a main port-city rival to The City of Venice no less) and will be really stupid to pay Ottoman tributary for 500 hundreds years! Additionally these port-cities such as Dubrovnik are autonomous and not even directly under the empire administration. I'd imagine the countries or states under the Ottoman empire will be even more integrated than that in term of taxations and contributions? It will be like asking UK to pay taxes/contributions for several hundreds years to EU even after the separation from the EU. Without a unified and strong empire you cannot do that even in today's world let alone in 16th century AD.
It seems that what EU (federation of participating countries) and USA (federation of participating states) practices of major trade blocs are not unlike the old empires, but the age of empires namely Imperial China, Byzantine, Mughal and Ottoman probably fare much better with their openness to trades.
The trade blocs as we know today was started by the Hanseatic League of the Holy Roman Empire that is neither Holy nor Roman nor an Empire [1].
[1]https://en.m.wikipedia.org/wiki/Trade_bloc