Reaserch

[Philosofire]

As you would in a world without multiple levels of research, you have to upgrade the forge and other buildings to unlock the ability to research certain units. Beyond that the only thing limiting your research is how many research points you have and your available resources. For example: With a Lvl 3 Forge, you could have Lvl 3 Lancers, Sentries, and Berserkers, if that's what you wanted. You'd still have 1 point left over, so you'd have to have at least a Lvl 1 Stable to put your last point into researching Scouts. With a Lvl 10 Forge, you can downgrade any units that you've put points into, much like how you need a Lvl 15 City Hall to be able to demolish/downgrade your buildings. Hopefully this helps.
Cheers,
Philosofire