2 Like a huge cave, hewn out of rocky cliff, 3 From whose rough vault the ragged breaches hung,

breaches > {?Broken sheets of rock}

4 Embossed with massy gold of glorious gift,

Embossed > Encased; richly decorated massy > solid gift > quality

5 And with rich metal loaded every rift, 6 That heavy ruin they did seem to threaten;

That > [So that] heavy > grievous, intense ruin > {The act of collapse, of a building, etc.}

7 And over them Arachne high did lift

Arachne > (Transformed by Minerva into a spider (Met. 6.1-145);
Ovid's story is retold and embellished in Muiopotmos 257 ff.)

8 Her cunning web, and spread her subtile net,

subtile > delicate; rarefied; crafty