General Antonyms:
* Interior: This directly contrasts the idea of a border or edge.
* Center: This implies a location of importance and established presence within a region.
* Established: This emphasizes the opposite of the wild and untamed nature of a frontier.
* Civilized: This highlights the contrast between the wilderness and human society.
Contextual Antonyms:
* City: If the context is about a physical location, this contrasts the frontier's remoteness.
* Peace: If the context is about the frontier's danger, this suggests a calm and safe environment.
* Familiarity: If the context is about the unknown, this suggests a place that is well-known and comfortable.
Specific Antonyms:
* Rear: If "frontier" refers to the front line, "rear" indicates the opposite end.
* Metropolis: If "frontier" refers to a sparsely populated area, "metropolis" suggests a densely populated urban center.
Choose the antonym that best fits the context of your usage.