To answer my question, I looked up the chapter on Ultima in Pritchard's Popular Chess Variants, and he wrote, "An immobilized king can be mated by its rival moving next to it." So, unless Abbott has said differently somewhere, I'll take this as the rule.
To answer my question, I looked up the chapter on Ultima in Pritchard's Popular Chess Variants, and he wrote, "An immobilized king can be mated by its rival moving next to it." So, unless Abbott has said differently somewhere, I'll take this as the rule.