I hope that homage is as okay in game design as it is in movie-making, as this is basically a mash-up of elements of my favourite games - a little bit Carcassonne, a little bit Pandemic, a little bit Mechaton, a little bit Dwarf Fortress.
Anyway, the idea is that the players (for some reason, in my head, there are three of them, but I suppose there's no practical limit) are competing to build citadels while fending off wave after wave of increasingly vigorous NPC enemy attacks. The way I had it in my head was that each player starts with a 16x32 Lego base plate, and has a pile of lego bricks to one side and a pile of cards in the middle. The players take it in turns to draw cards, and many of the cards will have improvements to the citadel that the player can build - increasing wall size or thickness, archer towers, gates, boiling oil, a moat, etc etc. For example, if you drew a wall card, it would have a number on it, which would represent the number of... what's the technical term? Nobbles that you could use to build your wall. So if it was a 4, say, you could use a 4x1 or a 2x2 brick. I imagine that further restrictions would be needed, like the maximum depth or height to which an element could be built.
Scattered in amongst these, though, are invasion cards. Every time one of these is drawn, an invasion force is generated by the game which lays siege to the players' citadel. The mechanics for this are a little bit rough in my head, but I envisioned the invasions getting stronger with each card drawn - like the outbreaks in Pandemic, or the bonus armies in Risk - and the strength of the army determining how much damage they could do to your citadel. I envisioned the damage as permanent, as the invading army literally tears down walls. I'm unsure how much of this to leave automated, though, as the other players could 'manage' the invading army, if they were given the framework to do so.
In any case, the aim of the game is to be the last player standing, as the increasing attacks outstrip your ability to rebuild.
Thoughts and concerns. - Obviously the invasion mechanics need to be worked out properly. - At the moment, the outcome seems to be based too much on randomness. Hoping that the invasion cards distribute themselves fairly seems like bad design, but otherwise, there's every chance that the outcome is decided purely by who has the most vile run of bad luck. At least in Pandemic that was bearable, as the bad luck was collective and only advantaged an NPC villain. - I was toying with having a world-building element. For example - the game starts with the players' baseplates on the ground. In addition to drawing cards, the players can also draw a terrain tile and place it somewhere in the middle of the play area. Invasion cards do nothing if drawn until an Enemy Stronghold terrain tile has been drawn and placed. Perhaps invasions are colour-coded to specific Enemy Strongholds, and always attack the nearest citadel, regardless of who drew the card? I can't think of any other way to tie this to the game, though, so it's off to the side for now. - Different types of invasion force. for example, an army of archers would be much weaker against walls that were high, while siege weapons would struggle against walls that were deep, and melee forces would struggle to deal with moats.