Base Warfare: Assaults

I've been playing for 100 hours or so now and finally got around to playing with stone construction. I am making a multi-tiered village, with each tier being separated by a defensive wall. But what's the point? My hold, the innermost circle, has a stone wall 2m thick surrounding it, but even trolls fail to break a block, let alone breach the wall. And base raids, as far as I've seen, are a nuisance at best once you build a stone wall. Keep in mind I haven't defeated Bonemass yet so if this changes in the late-game I apologize. So here is my idea:

A player-triggered super-raid, the Assault.

The player must build an Odin altar, which would be unlocked when they build a Stonecutter. I'm not sure on the exact resource cost, but something that would require some loot gathering (maybe some ancient bark, iron nails, and stone?). The player can then sacrifice the items required to summon the bosses to initiate an Assault. Depending on what items they sacrificed, a massive horde of that boss's raid type will appear (i.e. sacrificing Ancient seeds will summon swathes of Greydwarves, GD Shamans, and GD Brutes).

So why would anybody want this? Successfully fending off an assault would spawn precious items in the respective boss's "age" at the Odin altar. If one had just finished an Elder Assault, they might find Bronze ingots and Surtling cores. A Bonemass assault might spawn Iron Ingots and chains. Something that makes the Assault a worthwhile, albeit dangerous, endeavor.

As for the specific conditions of the Assault, they would be taxing, lasting between 8-10 minutes. Defending against the onslaught would require heavy fortification and active intervention on the players part. Furthermore, if the Odin altar is destroyed during this attack, the Assault will end and the player will receive no rewards. Enemies within proximity of the Odin altar would prioritize it over other structures, but would still target the player first, allowing people to kite enemies away if all defense has failed.

I debated on whether to include raids that aren't tied to a boss (i.e. "The ground is shaking"), but I'm not sure what loot they might offer to make them worthwhile.

Any thoughts/suggestions/harsh criticisms?