Hard to kill
One of the tricks about heavy, thick armor is that it’s hard to put a sword through. Or a spear, arrow, or much of anything. A pollaxe ought to do it.
This is reasonably well represented in dungeon gaming through the abstraction of Armor Class, in that it is more difficult for a character to to land an effective hit on a foe in thick armor.
Easy to Grab
On the other hand, many of the old fighting manuals focused on grappling foes in heavy armor. That’s because being encased in mail or 2 to 4 millimeters of forged steel doesn’t make you harder to grab (though it may make you harder to lift).
Dungeon Grappling enables this explicitly, adding a nuance to fights in your games that might not be present today.
Consider a guy with DEX 18, which is good for anywhere from a +1 to a +4 bonus depending on edition. Let’s go with the +4. In recent editions, wearing heavy armor caps that – in fact, no bonus at all is provided, but in plate armor you are AC 18.
The Grapple DC for this character, with the rules presented in the book? Grapple DC 10.
Coup de Grace
So now, the go-to method for dealing with heavily-armored knights is probably wrestling them to the ground so you can stick a dagger in a vulnerable spot.
Kinda like this:
Dungeon Grappling. For all your can-opening needs!