I just had a few ideas that I thought I would suggest, and hope that others would build off of

rare spawns

ironwood feller: a rare replacement to the ironwood defender, this appears similar, but carries a large circular saw at the end of his arm.

Type:Construct
attacks: smash (same as the ironwood sentinel), slash (swings the buzz-saw through the air)

fat cat: a rare replacement to any spawn of a spookat, they have a small wallet on their side and sport a monacle. drops more crowns and higher chance of material drop.

Type: Undead
attacks: tier 1:bite (same as spookat);tier 2: bullet(same as spookat), can raise attack of other spookats near it;
tier 3: same as 2, more bullets.

new monsters

Midas' Grip: a strange golden hand that propels itself across the floor, it attacks any that draw near it.

Type: Construct
attacks: all tiers: can hop into the air and smash the knight, causing significant damage and stun(quite slow), can touch other monsters, giving them a gold sheen and raising their defence (no other effect - no higher loot chance)

Kataclysm: a stitched up little kat with a fire in its eyes.

Type:Undead
attacks: normal: tier 1: bite(same as spookat); tier 2: bite (attack increase from tier 1); tier 3: bite (increase from other tiers)
Ignited( if a fire affect is used on the kataclysm, it ignites): tier 1: bite(normal), rush(runs across quickly, spreading fire to anything it touches and inflicting small fire based damage); tier 2: bite(same as normal), rush(increase speed and attack from tier 1), bullet(fires small fire based projectile; tier 3: same as tier 2, but with incrased speed and attack

bosses

King Midas & undefined stratum: A large golden head sits idley by the gate out of the ruins. as knights draw near, it opens its eyes and attacks.

STAGE 1: king midas summons 2 Midas' Grip to attack the knights, while the Midas' grips are alive, the king is invincible. Once the Grips are defeated, the king looses his golden hue(looks like stone) and is vulnerable to attack.
info: two guns mounted on the side shoot projectiles (the base of the statue can turn, so his degree of fire is 360). STAGE 2: the bottom of the base lifts up, revealing treads. King Midas now moves about the arena, chasing knights and firing at them, he also summons 4 midas's Grips, which still must be destroyed to attack the king directly. It is a construct
attacks: STAGE 1: bullet (2 bullets, 1 fired from each side); STAGE 2: bullet (fires 2 from each side, 1 forward and 1 away from the base of the boss), rush(revs up for 5 seconds, then races across the arena attempting to plow through knights)

Grimalking & the second undefined stratum: As the knights make their way down the narrow path, a pool of darkness forms behind them. with a roar, a gigantic black mass breaks through the ground and begins chasing the knights.

STAGE 1: the knights rush forward, avoiding spawned in Kats, and other monsters; as well as different environmental hazards and block formations; STAGE 2: a large arena at the end of the pathway with several torches(able to pick them up) the simple goal -- while defending your group from waves of enemies -- is to light the area by placing the torches in strategic areas to make sure the Grimalking cannot enter.
info: The Grimalking is a very large Grimalkin, it moves at about the same speed, is immune to damage, but can cause much more to the knights. It is an Undead
attacks: Bite (the same as Grimalkins, but MUCH higher damage given)

Stratums
I have no clue what the stratums for the bosses will be like. want to help me out here?

it should be noted that any effects or changes the monster types go through through the different tiers (i.e. move faster, more bullets) are reflected in their rare counterparts. ALSO I'm working on concept sketches, but don't be thinking they'll be too good, but they'll get the idea across. I don't know when those will show up.