First, take a look at the screenshot below.
It's an island, it's not RISK like, everything you see belongs to the player (no borders, no enemies, no nothing). Just one little cozy island that is fully controlled by the player from the start.
The game is a turn based steampunk style strategy game. Most of the time the player deals with industry and trade (irrelevant here).
But, sometimes an enemy empire (from imaginary "outside the map") invades the player's island. The player needs to defend (so in an abstract way it's a variant of a tower defence).
Any ideas how such "defensive battle" could be implemented? Any ideas, just brainstorming for now.