Near an (undestroyed) allied tower it you get 8 armour and 8 mr.
Resolve sometimes feels like it doesn't offer as much raw defensive stats as other paths offer damage outside of Aftershock's burst of armour/mr. Siegemaster is the perfect rune for this. Add it to the mid tier along side Second Wind, Conditioning and Bone Plating.
Second wind helps you sustain lane poke, bone plating helps you survive combo burst and conditioning for scaling. Siegemaster would give you the choice if you know you will have a hard lane pushing into you and help against dives. Would also help melee top laners from poke against ranged one, as they try and get caster minions.