In my last play-through, I made a huge mistake of not keeping my army up to date. I went with defending cities with one Legion unit. Some cities had Pikeman units, and others had ancient Warrior units guarding them. This happened because I was taking a pacifist approach to the game. I was trying to win the game through religious victory. To win with a religious victory, you have to convert all other civilizations to your religion. You don't have to capture or invade cities belonging to other civilizations. So, my army was only for defense. I never started any wars against other civilizations. After clearing out the barbarians on my home continent, I let my guard down. I didn't notice that Japan, my next-door neighbor, was amassing Samurai units beyond my borders. Their target was my capital city of Rome.
Some notes I made while playing Civilization 6 on the Xbox:
Builders only have 3 uses or charges. What this means is that after using their “Build Improvement” ability for the third time, the unit will be consumed and will disappear. You'll have to build a new one if you want to build more improvements. This is different to the Builders in Civilization 5, which stay in play for the duration of the game and don't have a limit on how many improvements they can build.
Archers are pretty useful with their ranged attacks. Useful in the sense that if they attack an enemy unit 2 tiles away, the enemy units won't be able to deal damage back to the Archers. However, out on the open field, they are a little vulnerable against melee units, so it is best to have them escorted by your own melee units.
I have found that having them garrison in a city makes them good defenders as they don't have to venture out of the city to attack enemy units; they can simply fire away from within safety of the city walls.
Keep the envoys and don't send them to other city-states until you really need to. – This one is actually a tip from Coney, who has played the game a lot on the PC.
Focus on sending envoys to city-states that are in close proximity to your cities. This is so that if you gain enough influence on them, they will not go against you when wars breakout.
You don't need to build Fishing Boats to build improvements on water tiles, instead you can just use the Builders. – Wasted a lot of turns on this, waiting for the game to allow me to build fishing boats >_<
Astrology is the early tech that gives you Holy Sites or unlocks buildings than can generate faith.
Be wary of constructing buildings as some buildings can only be built if you satisfy certain conditions. For instance, some buildings can only be built on a tile that is adjacent to a river.