First, when a city, etc.: the skeleton of this highly involved sentence is as follows: 'First, when a city shall be as it were besieged. . ., that then the people . . . should be disputing. . ., argues first a singular good will. . ., and from thence derives itself [i.e. flows on, proceeds] to a gallant bravery. . . .'
28 as his was who when Rome, etc.: this story is told by Livy, as an instance of the undaunted spirit of the Romans during the Punic war.
mewing properly means 'moulting.' Milton apparently uses it in the sense of 'renewing by the process of moulting.'
29 engrossers: wholesale buyers; here used metaphorically of those who, by curtailing the liberty of book-printing, would 'buy up' the stock of knowledge and dole it out as they thought fit.
30 he who takes up arms for coat and conduct: this refers to Charles I's exaction of a tax for the clothing and conducting (i.e. conveying) of troops.
his four nobles of Danegelt: a noble was a coin worth 6s. 8d. Danegelt was originally the land-tax raised by Ethelred the Unready to buy off the Danes; the word was afterwards used of any unpopular tax, here of Charles I's imposition of ship-money, resisted by Hampden.
In this unhappy battle: the battle of Newbury, Sept. 20, 1643, in which the advantage was on the whole with the King against the Roundheads.
33 vacant: i.e. open, unclouded.
addresses to his place: i.e. to his office. Falkland was Secretary of State to Charles I.
40 Phalaris: a Sicilian tyrant of the sixth century B.C., famous for his cruelties. The Greek poet Stesichorus was a contemporary of his.