Upgrading units:
- Warrior to Spearman: you sell old club, buy new spear and keep old people. Hardware is replaced (costs gold) but you keep experience (XP), promotions
- Frigate to Battleship: same. You sell old hardware, rush buy new toys and keep people, crew.
Rush buy:
- when you build stuff with hammers, you can say you build in emreror/king/state controled workshops, factories
- when you rush buy, you can say you hire subcontractors, buy on free market, etc ..
Marines:
- destroyer can capture city
- Marine can capture city and then (after some healing) step out of city and attack, protect, tank vs counter attacking enemy melee and ranged units. DD can't do that.
- any land melee unit can also do that, but marine is a bit better when it comes to 'water'
- not 'must have' units, but not useless