bastion

As nouns the difference between bastion and rondel

is that bastion is bastion while rondel is a metric form of verse using two rhymes, usually fourteen 8- to 10-syllable lines in three stanzas, with the first lines of the first stanza returning as refrain of the next two.

As nouns the difference between bastion and demilune

is that bastion is a projecting part of a rampart or other fortification while demilune is a fortification constructed beyond the main ditch of a fortress, and in front of the curtain between two bastions, intended to defend the curtain; a ravelin.

As an adjective demilune is

in the shape of a half-moon, i.e. semicircular.

As nouns the difference between bastion and moineau

is that bastion is a projecting part of a rampart or other fortification while moineau is a small flat bastion, raised in the middle of an overlong curtain.