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.

OK, I will start then :)

I would use a sort of "liquids" behaviour and "frontlines" system (instead of units).

The enemy (AI empire) lauches an invasion and selects 3 neighbouring coastal provinces that seems the weakest. Land forces of infanry land there while ships and zeppelins bombard fortificaztions in these provinces. Once a province is conquered/controled, the AI move forces to adjacent weakset province and proceed, all provinces that have no player forces neighbouring would leave the province and gravitate toward provinces neighbouring player forces. This will continue (choosing the weakest neighbouring province path) until all provinces are conquered (or the player agrees to peace and pay the tribute/consequences (there would be probably no game over after military defeat, or at least not always)).

The player has very similar ground infantry regiments. These station in coastal provinces only by default. The player can also build fortifications in provinces.

When a battle occurs and player forces are losing (almost guaranteed at the beginning of an invasion due to heavy concentration of enemy forces in a few coastal provinces) the force is not simply destroyed. Only part of the regiment is lost, the rest retreats to the weakest friendly neighbouring province strenghtening it (forming a frontline of a sort). And then it all repeats until one side is wiped out or the player or AI agrees to peace favourable to one side.

The player forces from distant provinces (not in danger) would slowly move towards frontline (railrad system is important here! if the player built it it's much faster).

So far it's all automatic (no player decision, except of decision when to builf fortifications prior to the battle and preparing railroad infrastructure). I',m not sure if it's good or bad :) But I think I clould also add some sort of "orders". Like the player puts markers "gather here" or "leave from here" in provinces as he/she pleases.

There could be also some special forces (non infantry), like zeppelins, elite divisions of steam tanks, etc which could be manually deployed to provinces.

I could see some sort of growing creep for invasions. All the enemy forces start at the invasion points and then spread out from there. Invading a hex flips ownership. A hex is invaded if there are no armies in it or they are all defeated in an attack.

Forces:

Infantry 1/1 = Infantry stack and on their turn a stack either splits and attacks an adjacent tile or moves to an adjacent tile. if a 2 infantry stack attacks 1 infantry stack then the defending stack is destroy and the 1 remaining infantry invades.

Zeppelins 3/3 = Zeppelins can jump two spaces allowing them to move over enemy forces without attacking them.

Cannon 2/1 = Stackable, First strike, range of two. Cannons can invade adjacent tiles or at start of turn attack any enemy in range.

Giant Mechanical Spider 5/5 = Massive attack. Giants spiders attack and invade all three hexs in their attack direction.

The whole thing would play out automatically. Like some kind of combination tile based strategy and hexxagon.

We got quite a lot of upvotes, it seems this whole "automatic battle" thing has something appealing in itself :)

But I'm a bit worried about player's decisions (lack of these). Maybe we could make it partial automatic (all the boring parts of shuffling infantry to the front line) and partial manual (all the cool, interesting, strategic choices)? Like, the infantry is automatic, but special units (zeppelins, ornithopters, heavy artillery, mechanical spiders) are directed by the player?

Subturns/battle turns

I got another idea, making "higher density" turns during battle. I mean, when a war erupts he turn system is becoming annoying (you move units then click end turn and have to deal with economy decision when you just want to finish the war!)

So, when a war starts, you get "battle phases" (another counter of 1/12 battle phases and another button "next phase", below end turn button). You play as usual, but the "End Turn" button is grayed out until you finish all battle phases. Then a next turn starts and if war was not concluded you get another 12 battle phases.

As an added extras, the battle phases don't need to be identical (the first phase allows only artillery actions, every 5 phases slow units can move and every 3 phases fast ones).

Invading a hex

I am a little worried though, that there may be no way to "win", and all you can do is "not lose".

Why do you want combat without expansion?

It's just a subsystem for a game that is not primirily a wargame.