The wiki has some frontage/terrain info:
http://www.ageod.net/agewiki/Frontage
And Narwhal has covered the same in one of his AJE Beginner's Corners:
https://forum.paradoxplaza.com/forum/index.php?threads/the-hero-the-traitor-and-the-barbarian-an-aje-pbem-beginner-aar.632034/#126
Beginner's Corners
Structure of units
Unit statistics - Basics
Command Points
Activation
Posture and ROE - Basics
Taking structures - Sieges, blockades
Leaders
Unit statistics - Details
Supply
Resources : Money, Engagement Points, Prisoners
Engagement Points
National Morale
Unit production, replacement production
Combat I - Frontage [IMPORTANT]
Combat II - Fire phase
Combat III - Melee phase
Combat IV - Targeting