Part of the 0 A.D. Gameplay Manual
Quickstart Guide - if you've played other RTS games
This guide aims to give you the basic facts you need to get into playing without boring you with the details and concepts which are the same in all RTS games. If this guide doesn't give you the answer you're looking for you might want to read through the Basic Gameplay page, or another of the pages in the manual.
The main difference is that some units have the dual role of being both workers and soldiers. They are called Citizen Soldier and are the most basic soldiers, a few of them can be built from the Civic Centre (comparable to a Town Center in Age of Empires etc) and the rest from the Barracks.
0 A.D. uses resource dropsites, like the original Age of Empires games and Empire Earth.
Damage in 0 A.D. comes in three types: hack, pierce and crush. Units have different armour for each type. In addition, unit attacks may be of multiple types simultaneously (e.g. a Hoplite does both pierce and hack damage). Therefore, you may see some units do different amounts of damage to different enemies.