Dominions 5 has a multitude of nations from different cultural backgrounds (Norse, Greek, Roman, Asian, African, Arabic, …), spread over the Early, Middle and Late Ages.
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While these nations may be thematically distinct from one another, in terms of gameplay, most can be categorised into various archetypes. Some nations fit into several different archetypes, and many strategies may favour one of these over the others.
Underwater nations start beneath the waves. They frequently have access to powerful Water and Astral magic. Underwater nations may be difficult to attack, but on the other hand, these nations can struggle to get a foothold on land, and cannot recruit national units in inland forts. Underwater nations lack conventional ranged weapons, and rarely use iron, though some have exotic alternatives to either or both of these. Underwater nations may have limited but highly valuable recruitment options in coastal fortresses.
Closely related are amphibious nations - these start on land, but have special recruitment options in underwater fortresses. Most underwater nations have either disappeared or been driven onto land by R'lyeh in the Late Age.
Blood nations depend to varying degree on collecting blood slaves and sacrificing them to call forth demons from Hell. As many of the more powerful blood spells come later in the game and they have to build up their blood economy, they often (but not necessarily) have a weaker early game, but a very powerful late game. They are more powerful in the Late Age, where the population is generally larger, but there are powerful Early Age blood nations as well.
Communion nations rely on their large mage corps using communions to pool their power to reach high levels of magic and carry the battle. Many of the nations have a Mediterranean theme to them (e.g. Greek, Roman) and often field massive armies of Legionaries and Hoplites led by mages. Most of these nations are composed of humans or similarly fragile races.
Elf-like nations have long-lived and glamoured units, often fielding powerful lightning magic and dangerous cavalry. Due to their stealthiness, Elf-like nations are excellent raiders who are able to mount massive preemptive strikes and raid enemy provinces at will. You might want to take a look at the elf strategy video by Perun.
Like the difference discussed between Giant and Half-Giant nations, Stealth Nations are similar to Elf-like, as most of their prominent units have stealth. They can Raid like Elves but lack most other Elven qualities. Additionally, Raids sent out by Stealth Nations are easier to detect than Elven raids. Stealth Nations often have other traits to contrast them with Elves. You might want to take a look at the elf strategy video by Perun.
These nations can recruit mostly flying units, which makes them very mobile and dangerous to fight against. Due to the siege bonus flying units receive they are excellent in sieges, breaking down or holding up fort walls with vigor. You might want to take a look at the flier strategy video by Perun.
Nations with mostly Half-Giant (size 3) or Giant (size 4-6) recruits. They field fewer units compared to human sized nations, but each unit is a lot tougher and stronger. You might want to take a look at the giant strategy video by Perun.
If you want to use specific magic paths, you get an overview over recruitable national magic here:
These nations can recruit powerful Giant mages which, when equipped with specific items and spells, can single handedly take on enemy armies (e.g. Tartarians).
Nations that have a native temperature preference or prefer temperatures due to their race (e.g. heat for cold blooded, cold for undead).