[quote] from user below: "I have collected 1D skulls and still can't upgrade carl to lvl11. is it a bug, or have I maxed its lvl?" [/quote]
You have to upgrade to level 9, then collect as many skulls you can, then buy MAX for carl to get him past level 10. You can only upgrade him once. So you can get from 9 to 12, or 9 to 15 etc but after that you can't continue to upgrade him. Hope it's clear.
The skulls needed for each level:
That's looking awesome! great job!
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