7. Keywords and effects
Shield
Units that hold their ground
is Assault turned around: a unit with Shield has +1 while it's a defender. Shield 2 gives +2, and so on.
Only while defending
has 2 . When your opponent moves units onto its battlefield, it defends with 3, and needs 3 damage to die. When it's the one moving in, it's an attacker, and it's a 2 again.
So a unit with Shield is best left where it is, keeping a battlefield you've already taken. Shield from more than one place adds up, the same as Assault.
Note
A unit can have both. One with Assault and Shield gets its bonus whichever side of the fight it's on, but never both at once: a unit is an attacker or a defender, not both.
Try it
Keep a battlefield with a unit that has Shield, and see how much it takes to get it off.