2 And all his armour seemed of antique mould,

mould > style, design

3 But wondrous massy and assured sound,

massy > solid, weighty (cf. Arthur's shield at 107.33:6)

4 And round about fretted all with gold,

fretted > interlaced (another heraldic term); adorned

5 In which there written was, with ciphers old,

ciphers > figures, lettering

6 Achilles' arms, which Artegall did win.

Achilles > (The great hero of the Iliad, so linking Artegall with
Troy and hence Brutus: see 210.9:6 ff., 309.38:8 ff.)