Yes, the noun 'octagon' is a countable noun, the plural form is octagons.
Examples:
An umbrella is a common form of an octagon. (singular)
The pattern is a series of octagons in traditional colors. (plural)
Yes, property is a countable noun.
The noun 'octagon' is a concrete noun, a word for a physical thing.
Yes, castle is a countable noun: We have two castles, one castle in the countryside and one castle on the ocean.
noun
countable
The noun 'hill' is a countable noun. The plural form is 'hills'.
Shark is a countable noun.
Prawn - prawns is the plural - is a countable noun
Transport is both countable and uncountable as a noun.
Yes, property is a countable noun.
The noun 'animal' is a countable noun. The plural form is animals.
Yes, the noun 'marriage' is a countable noun. The plural noun is marriages.
The noun meeting is a countable noun; for example: We have a meeting this afternoon. We've had two meetings already this week.
The noun 'desert' is a countable noun; the plural form is deserts.
The noun analysis is a countable noun; the plural form is analyses.
The noun utensil is a countable noun; one utensil, many utensils.
Countable