Brad, I can confirm that the seven warmachines will NOT erode the fortifications. The rest is one man's opinion:
Since you are dealing with a castle there will be a 60% negative adjustment. Since you lack the minimum 2500 troops add another minus 20%. Presume you have as commander a pumped up Argeleb at 60, plus he has his two command artifacts #5 (add a random number between 1 and 35) and #38 (add another 1 to 20 points). If you got incredibly lucky on the artifact random roll you now have a 35% chance of sieging the place.
However, I did once see a small Noldo army besiege the capital of a dropped Rhudaur (back in the GSI days, before Harley replaced everyone who drops, no matter when or how). So that may enter into the formulary.
ยทยทยท
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