The analyses of a text #4

The narrator recalls his adolescence.
He was an apprentice for Mr. Spenlow.
He and Mr. Spenlow's only daughter Dora fell in love with each other, and he got married to Dora.
Today He invited Traddles to dinner.
Jip is Dora's pet dog.

.................................
There was another thing I could have wished, namely, that Jip had never been encouraged to walk about the tablecloth during dinner. I began to think there was something disorderly in his being there at all, even if he had not been in the habit of putting his foot in the salt or the melted butter. On this occasion he seemed to think he was introduced expressly to keep Traddles at bay; and he barked at my old friend, and made short runs at his plate, with such undaunted pertinacity, that he may be said to have engrossed the conversation.
However, as I knew how tender-hearted my dear Dora was, and how sensitive she would be to any slight upon her favourite, I hinted no objection. For similar reasons I made no allusion to the skirmishing plates upon the floor; or to the disreputable appearance of the castors, which were all at sixes and sevens, and looked drunk; or to the further blockade of Traddles by wandering vegetable dishes and jugs. I could not help wondering in my own mind, as I contemplated the boiled leg of mutton before me, previous to carving it, how it came to pass that our joints of meat were of such extraordinary shapes - and whether our butcher contracted for all the deformed sheep that came into the world; but I kept my reflections to myself.
[David Copperfield by Charles Dickens]
1. I'd like to know why it is "at his plate," not "to his plate."
2. I'd like to know what "castors" means.
3. I'd like to know "by wandering vegetable dishes and jugs" modifies "blockade."
4. And I'd like to know what "contracted for something" means
Thank you in advance for your help.