Well, one of the most important countries in my own homebrew setting is based on Rome, literally.
Romans from a *counter* Earth, so wasn't same as our Rome, came to an area, the remains of a mighty, but evil Empire decimated in the catastophic war that wrecked the world a millenia before.
Now, these Romans had soemwhat different ideas than "our" Rome. The Emperor was the head of the military and picked from within it by a random lottery from skilled and honourable centrions and officers. His job is to ensure the legions work well, and over see campaigns.
it is considered vile treachery for the Emperor to try and gain political power, or the senate gain military control, by law and custom, and this keeps things on a more even keel. It's worked mostly well for 4 millenia (bar a particularly malign hiccup that enforces the point).
The empire in times past was more brutal and expansionist, but for a long period it has been more benign, let once conquered naitons gain freedom, and is now looked on as a very stabilizing trade and military partner for the region. old grudges are mostly long gone in the face of mutual threats, trade and appreciation.
The Empire widely uses and encourages magic, but also has very strong laws and a codified police force, external agents and bounties are common place (so there's plenty of scope for adventurers, including detectives).
The basic law though, is that no one is allowed to carry a weapon beyond a knife or staff, without permits, and excessive magical violence is also illegal, obviously (ie, Fireball in town = gonna get you roasted in return).
magic is used ot help build the canals and roads that have greallty improved trade and food production, and the military has its own magical academy, orphans are taken in, trained and become support units for the legions.
The legions are armed/armoured in adamantine, over the millenia, the empire's adamantine mines have provided the army with superb advantage, and since the adamantine is practically impervious to damage and wear, it has accumulated. From being originally only a thing for officers, every legionary has such now, that with their training and magical and other support units makes them greatly feared.
A typical century will have 50 fighters lvl 2 to 4, adamntine shortsword and banded mail, large shield, javelins. 10 archers or slingers (some of whom will be scouts, so rangers or rogues), and 5 centurions (lvl 4 to 9 fighters), 5 spellcasters of lvl 3 to 7, plus one tribune or senior centurion, and 10 calvary of one specific type (light, archers or heavy). They are definatley NOT "warriors" or mooks, they are well trained soldiers.
many legions have specific make ups, and duties. Such as dwarven mountain regiments, heavy assault infantry (elites), etc.
The Empire does of course, have problems.
The ancient war left terrible regions of magical chaos and poison, from which threats may erupt, moutainous regions from which orcs etc may attack (so the empire has a line of defencive walls etc, think Hadrian's Wall D&D style), and the once Empror who became a tyrant, still lives as a vampiric lich and always dreams, and tries, to take his throne once more, which has lead ot catastrophic wars.
Great arenas mostly have races (Which are pretty huge and may have rocs, wyverns, magic carpets, as well as chariots, mastadons and all kinds of strange vehicles and beasts), gladiators rarely fight to the death any more, unless condemned criminals, and most serious criminals are sent to the adamantine mines, which is a slow death sentence as the super hard dust eats away at their lungs...of course, there are large thieves guilds, it's a rich palce indeed! Hence the need for having a true police force, not just a city watch (many wealthy districts hire mercenary companies for extra security, and each major town will have a legion stationed in the town or close by, on a rotated duty, as well as as civic militia for major threats).
The main city is a vast place, sheltering inside the partially remaning, brobdignagian-sized defencive works of the long destroyed empire.
Two elite legions guard it at all times, inside with more near by,
"subura", the place most ordinary folks dwell is a rough place in the nastier wards.
The city has vast warren underneath it of anicent ruins, though some places have been cleared down to bedrock for safety after fires or monster invasions.
Pretty fun place
Had a long running campaign where the PCs were "vigeles" (police, in this setting, unlike Rome, per se).
One fun adventure was discovering the local pub they liked, the pies they ate, well, they came from a butcher, who was a yuan-ti, and well..."long pig" tastes good, don't it?