This is horribly off-topic, but please tell me more about this !!!
What game level, map type, turn/year, tech level, who vs who etc.
I was astonished - never seen a game like it. Never underestimate the AI's desire to beat you when desperate.
I was Darius, Emperor game on random Huge map that turned out to be islands. Except for the ever-faithful Isabella (she and Askia have turned up in
every one of my recent games) everyone now hated me (as I got closer to victory, my long-time allies the Inca, Korea and Arabia had successively denounced me and warned me of barbarian invasions), though only the Ottomans and Songhai were nearby. I'd had a continent/big island to myself (after occupying the three Songhai cities that had established a foothold) for most of the game, and had gone the science victory route. I had one spaceship part left to complete (which was building in Susa, on another continent), and had been at war with the Ottomans and Songhai (who declared together) for a while. Usual stuff, and not worried since, after all, the AI can't handle naval invasions - the Songhai had made some reasonable landings given their low tech level, but had no chance when throwing infantry against mech, and did occasionally lead with artillery (oops). But their main island was only a single hex of water away.
The Ottomans were more distant but still my second-closest neighbour; they'd sent a few token waves of infantry against Ecbatana (off my main island, and the closest city to them) without effect - usual stuff.
Then the 1960s hit, and my spaceship neared completion. Suddenly 4 destroyers appeared escorting 2 artillery, 2 tanks, several infantry and a cavalry right off the coast of Pasargardae. As an island map, where I hadn't had a major war (only a minor one against the technologically weak Songhai) I hadn't invested much in military, and had only one destroyer to see off marauding ships. Due to the lack of conflict my few units hadn't got many promotions (I had one mech inf with blitz, that was it). I still eliminated most of the attacking melee units, but couldn't quite relieve Pasagardae in time.
No problem, the Ottomans had run out of attackers, they couldn't hold the city. So I ignored the usual "give us everything" peace treaty offer (in retrospect, I was fewer than 10 turns from victory so should probably just have signed, as long as Susa wasn't part of the deal). Naturally, next turn more destroyers with a whole bunch of anti-tank guns and infantry showed up outside Persepolis, the one city (other than Susa) I couldn't afford to lose. Fortunately, with endearing AI inability to understand victory conditions, the giant fleet decided to turn round and attack Ecbatana, which was undefended. It didn't hold out as long as I'd hoped, but for some reason the large attack force stayed over there and didn't come back towards Persepolis, which was just hit with a few small waves of AA guns, anti-tank guns and bombers based in Pasagardae (another first - I've never seen the AI use bombers, only fighters and nukes. Plus they'd have needed to rebase to attack from Pasargardae, and I wasn't aware the AI could handle rebasing) which my army could hold off while the Ottomans used ground forces that should have been attacking my capital to attack Taghaza (with Songhai air support from fighters based in Gao).
While all this was going on, the Songhai did something very interesting - they repeatedly used fighters and artillery to bombard Tombuctou from Gao (my only ranged unit had been lost in the defence of Pasagardae, which it was garrisoning, so I only had city militia as defence), and it's not unknown for the AI to do things like this pointlessly when it has no way of capturing the city (I had a game once where Mongolia did that indefinitely to one of my CS allies, apparently unaware that all their melee units had been destroyed). What I didn't expect was that they would have an anti-tank gun lurking out of both sight and attack range (after all, invasion forces normally sit happily within the city radius where you can shoot them) - the turn Tomboctou's defences were reduced to 1, this thing just came round the corner and attacked the city from the sea.
In the event, Susa was never attacked and both Kinkia (closest rail link to the capital on the coast facing Susa) and Persepolis survived (although Persepolis was under siege and weight of numbers and my absent-minded tendency to throw modern armor at AT guns had seen off my army) long enough for the stasis chamber to get there and complete the spaceship. My space victory ended up feeling rather like the remnants of the Persian people fleeing from the burning remains of their empire...
If the AI had played only slightly better (attacking Persepolis with everything rather than hitting Ecbatana, and/or had the Songhai allied with Bucharest - on my island and with several mech inf, as well as naval units on the side of the island the stasis chamber had to enter from - instead of distant Singapore) I wouldn't have held out long enough to get the victory. As it was its choice of target left something to be desired, but for a rarity its actual combat could hardly be faulted.
Naval invasions, coordinated attacks by naval, air and ground units, allied AI forces coordinating attacks on the same targets, sensible use of air power (those annoying little fighters kept attacking cities - but if they spotted a mech inf in the open with a couple of remaining hit points, you could bet they'd switch targets and that mech inf would be no more. When coordinating attacks from 6 fighters based in Gao and an Ottoman bomber or two, even attacking cities with fighters makes sense), even a successful naval assault against a weakened city directly from the sea (and, admittedly, several of the usual pointless suicide assaults from embarked units by the Ottomans). Come to that, an unexpectedly sensible choice of first target (that first invasion was nearly stalled against Pasagardae - it would have been wrecked if sent directly against Persepolis. But it gave the Ottomans a well-placed beachhead on my island from which to support later attacks on the capital, as well as the pointless but effective invasion of Taghaza). I don't recall the Civ 4 AI pulling off stuff like that; for Civ V it's amazing.