Castra sunt in agro and boni sunt filii amicorum are two very different cases.
For the castra one:
Castrae does not exist. The singular form of castra is castrum, but then it doesn't mean "camp" anymore.
There are plenty of times when sunt should be translated as "is", and when "are" should be translated as est:
Hae litterae sunt - "This is a letter (epistle)"
Hae scopae sunt - "This is a broom"
Hae aedes sunt - "This is a house"
Haec castra sunt - "This is a camp".
"These are glasses (spectacles)" - Hoc est specillum
"These are pants" - Haec est braca (can also be hae sunt bracae - it's a rare word either way)
"These are scales (for weighing)" - Haec est libra.
This is just a matter of the Romans seeing something as plural (a camp is seen as a collection of tents, not a single unit) which we see as singular, and vice versa.
For the filii one:
As a mental exercise, what would filii be if it were plural?
mihi iussa capessere fas est