There are two modes of defense in Echelon, active and passive.
Active defense applies when the defender is aware of the attack and able to act. Both combatants make attack rolls and the higher wins. Ties mean both combatants win (and lose). The winner applies damage to the loser if possible. A combatant can only cause damage if there are means to do so – a ranged attack against a sword wielder likely means the defender can prevent damage but not cause damage. Of course, being Echelon, many high-tier sword wielders do have means to cause damage at range. Active defense is limited in how many times it can be used each round.
Passive defense can always apply when the target is unaware of the attack, unable to act, or chooses not to defend. A defender might do this when passive defense is better than active defense, or the defender wants to conserve active defense for a more dangerous target). This is rolled the same way but does not allow a counterattack that causes damage.
Armor is passive defense but is not linked to tier. Instead, die sizes are chosen by coverage, and number of dice by grade or nature of the armoring material. Minimal applicable coverage is d8 (piecemeal or patchwork armor), d10 (standard, such as chainmail covering head, torso, and thighs, while leaving much of the arm and leg exposed), and d12 (full armor). For normal armor the number of dice go from 1 (boiled leather or quilted), 2 (chainmail ,hide, or light scale), and 3 (steel plate, reinforced chain, etc.). Higher grades are possible: ‘dwarven made’, special materials, and magic could all increase the number of dice. Mixed armor provides incremental benefits between these: you can add a steel breastplate (3d10) to a full suit of chainmail (2d12) to make ‘half plate’ that is 2d12+1d10.
All creatures have natural armor, starting at Td6 (that is, a number of d6s equal to their tier — without other armor, Expert creatures have 3d6 ‘armor’). This seems contrary to the ‘coverage and material’ considerations of normal armor, but account for natural resistance to damage such as bones (yay skulls and ribs!) and relatively unimportant injury sites (“it’s just a flesh wound!”).
Natural armor beyond that — leathery hide, chitinous carapaces, shells, scales — upgrades this as normal. A horse’s hide is more or less full coverage (d12) but not very tough (one die), so a horse’s natural armor might be 1d12+2d6. Heavy barding (limited plate and a chain ‘skirt’ — 3d8+2d10 = 2d10+1d8) might increase this to 1d12+1d10+1d8.
There will likely be talents that grant varying degrees of natural armor. The dragon cornerstone comes to mind, likely granting ‘scaly hide’ that bumps natural armor right from the start (Td8). I expect to have a ‘natural armor’ or ‘tougher hide’ talent that improves this, and there can be capstone talents that likewise improves natural armor. You could find yourself facing a legendary dragon with 7d12 armor.
Well, at least it’s ‘passive’. It might be hard to injure the dragon, but at least you’re not running risk of getting hurt just by trying to hit the dragon. Except for that pesky ‘fiery aura’ or ‘caustic miasma’ talent, or the dragon taking personal interest and engaging in active defense instead.
There are two basic models used for armor: hit avoidance (AC in most flavors of Dungeons & Dragons — better AC means you get hit less often) and damage reduction (static or variable amounts deducted from damage — you get hit as often as someone unarmored, but not as hard).
Neither of these really worked for me in Echelon. The differences in contested rolls are almost always in the single-digit range, limiting how much hit avoidance is possible, and the damage done is often quite small as a consequence, making any amount of damage reduction powerful.
I think splitting defense into ‘active’ (you can hit back, but only a limited number of times) and ‘passive’ (you can’t hit back, but it’s always there) handles things. There are benefits to using either one depending on circumstances, but either way you are never quite defenseless.
I do notice that the baseline passive armor (Td6) is the same as untrained unarmed active defense (Td6) in terms of avoiding harm. I’m pretty okay with this — even when you know how, you can expect to bleed when attacked with weapon. Those without training being just as vulnerable to harm if they fight back as when they don’t is not out of line — but active defense allows you to try to hit back.