Yellow never gets the chance to claim his rooms and portal. Is an easy fix: Make a bridge behind the heart to the other bank so the imps claim it even if the CP doesn't dig the wall.
Also, you can make the wall impenetrable from the south by making it like this:
Code:
____________________________________
yellowwallyellowwallyellowwallyellow
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white tiles white tiles white tiles
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whitewallwhitewallwhitewallwhitewall
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So by adding a space of white floor in between. Like that only the yellow keeper can dig out to the south.
This is what's difficult about making levels, it is not about what you want the player to do, but about all the shit he can do and it may never break.