His plan was to kidnap the king's children and kill both of them after they tell him when the king was going to return. After he killed them, he would put explosives into the harbor and blow-up the king himself. Most vegetables find him too powerful due to the fact that he has an iron leg and a peg-leg, also due to the fact that his arms are made out of metal and that you can see them, unlike the rest of the Veggies who have arms that you can't see.
He also wanted to destroy three Veggies: Elliot, George, and Sedgewick...but since they came from the future, not the 17th Century, he didn't know about them until the King was almost returning, so killing them wasn't going to be as easy as he thought. Robert was banned from Monteria for years, giving him nothing to do except "dabble in the mechanial arts" In his exile, he built his Barrol-Robot Carrier and used Black-Powder to make highly powerful explosives.