This strategy is focused on early medieval aggression using the special bonuses of Spain: galleons from starting with Navigation and the +1 naval bonus (medieval). The attack force I am suggesting is a vet galleon fleet + vet legion army giving you an attack of 9+6=15 and a defence of 4.5+6=10.5 against counter attacks -- both of them are better than an Arab horse army and we know how deadly that can be. Plus with the Spanish you also have a 3-move mobility thanks to the galleon fleet. Now, how do we pull this off ?
First of all do not walk with the settler whale hunting. You may move 1 tile if you can still get 2 trees and access to water, sometimes it helps to get closer to a barb so that you can get it before it grows to 2 flags in 3500BC. You might want to avoid being next to a mountain as that can give away your location to your opponent even through the fog. Having 2 trees is crucial for this strategy so do not compromise that! If you can have 3 with 2 grass is even better but its rare. Another aspect to watch out for is to be able to have access to 2 different bodies of water if you are on a narrow land strip -- that could be very helpful to accelerate your exploration. I would even sacrifice food for that (as long as I get the 2 trees).
Once settled in 4000BC, you set your 2 workers on trees and build at least 2 warriors (5 turns), possibly more if you have more directions to explore or fear and early rush (impi or horse). It is often worth to spend some gold to rush a 3rd warrior as soon as you get your first gold find (exploration, barb or friendly hut). This is pretty standard so far. Now, for the twist: after your warriors, do not grow or tech, just keep your workers on the trees and start building a galleon while your warriors explore and collect gold. Once you reach 100g and get the free settler, rush the galleon and use it to ship the settler to a whale spot -- either island or mainland shore if you spotted one near. When you got your galleon, I suggest to leave its militia in Madrid fortified (preventing walk-ins or galley drops) and pick up one of your warriors with the galleon instead. That way you can still pickup any artifacts or huts you may find on island, but you also have the capability to take out barbs on islands or galleon-drop into an empty enemy capital if you happen to find one. The militia at home can alert you to any incoming threat with its 2 tile scout range, so you have time to rush a real defence when needed. Once your first galleon is out, keep your workers still on the trees and work on a second galleon or set your city to bank hammers on a wonder (EITC).
Exploration and building up your attack force:
You will need to have bronze working and iron working, a barrack in Madrid and build 3 galleons and a legion army (or more). That's 190 hammers which sounds like a lot, but you will rush some of it with gold and should be able to get a 2nd or 3rd city produce some as well. You can often buy bronze working from an AI for 20g, so do not start teching it, instead set your whale cities on gold to expand as fast as you can. You need to get 4-5 cities in ancient ASAP to have enough trade and production power. Your whale cities should grow to 3 pop, then spend 2 more turns to bank 8 more food and then rush a settler so they can grow back to 2pop next turn. If you can buy BW form an AI for cheap, do it and switch Madrid production to barrack, otherwise push out a 2nd galleon, then bank hammers on EITC -- do not build a 3rd galleon before the barrack is ready, you want that to be veteran. If its getting late (your hammers are piling up) and you still could not buy BW or obtain via galleon-drop, then tech it with all your cities, you should have at least 3 at this time, so you can get it in 2 turns max. Once you reach the point that you either have 4 cities,
3 of them with 3 pop plus Madrid with 3 pop OR 5 cities with 1 of them 3 pop the other 2 pop, then you set them all the trade (11 pops = 22 beakers) and tech one of the 20 beaker techs -- check the tech tree, if 3 of them are taken (no green 1 beside it) choose the 4th one. This way you should back-fill all the first level techs taking you into medieval to get the +1 naval attack bonus. Then you can research Iron Working in 2 turns, while Madrid is banking hammers again for the legions. Alternatively, you can do that earlier once you have BW and the barracks ready and the 3rd (veteran) galleon complete and you do not want Madrid's production to run idle. If your expansion goes faster than the production in Madrid, because you found good whale spots close by maybe with some extra food for faster growth (whale+grass can shave off 2 turns from the grow->bankfood->settler->regrow cycle, instead of 8 turns you can do it in 6), then you can build/rush a second galleon from a different city which has some trees. It is good to have the 2nd (non-vet) galleon out as soon as possible to speed up exploration, you may find some artifacts or a galleon-drop target. For this purpose, you should also switch the militia of the second galleon with your 2nd warrior, and leave the militia either in another mainland city if you settled such, or on a choke point. Exploration with the galleons is crucial not only to find barbs/huts and artifacts, but also to find out where are your enemies, so you can plan your attack ahead of time. Once you have your veteran legion army, pull together your 3 galleons (1 of them veteran) and form the fleet, your Spanish Armada is ready to attack. You may want to take along some archers for the ride, they can help take the heat off counter attacks and you can also leave them behind in the newly captured cities.
In terms of economics, it is best to complete the barrack and the 3 galleons while still in ancient as you will rush them at least partly and the rush cost goes up 50% (from 2gold/hammer to 3gold/hammer) once you enter medieval era. The legions are cheap you can even hammer them out completely, so you do not need to squeeze in the 30 beaker IW research in ancient. In fact, you could research that as your back-fill technology, since you will have bronze working already (need that for the barracks in ancient).
Naval supported hit and run attacks:
Your attack force should be ready for action just about the time when the initial exploration and rush-fights are over and the other civs are busy expanding or teching, sometime around 1500-1000BC. This time, you should have at least 1 militia in your fleet, so you can see what you are up against. Previously when you were exploring, you were sailing along the shores to see what's on the land, but once you form your fleet and going in for action, you should move around in deep water, so that your foes do not see your fleet approaching. Time your moves such that you arrive to the enemy shore beside the city in the last few seconds of the turn-timer running down. Land your troops to the attack position in the last second, then as the turn is over and the next turn starts, you attack immediately -- this is called the "double move" since you effectively made 2 moves (landing and attack) with no time left for your opponents to react in between. Nasty tactic, and very effective, but there are some caveats. You need to make sure all your cities have their work orders and NOT finishing a building, settler or wonder, units are OK, they just start making another without asking what to do. You should also make sure not to complete a tech in that turn as the advisor pop-up asking what to research will delay your attack giving chance to your opponent to make a move or rush another unit. Against stronger defence or multiple single units, you may need multiple legion
armies. Good thing, those are cheap, your major expense in this campaign is the barrack+galleon fleet (160 hammers), and those will not die in the attacks, so even if you need to make extra legion armies to keep up your invasion, that should be easy. This is an important difference compared to horse or knight rush tactics. Here, your major investment is safe what you can lose (legion armies) are cheap, while horses/knights/cats are more expensive. While your fleet is out there attacking, Madrid should produce archers and legions, both for its own defence as well as reinforcements for the attacks. The rest of your cities can work on long term goals, expansion, research, gold. You might win the whole game with this type of attacks, but do not count on that! Look at it as a great way to take a few cities here and there, boosting yourself while slowing down your opponent,
also forcing him to build more defences and counter-attacks in other cities. If you see that a city is strongly defended, do not hang around too long, just move on to another easier target.
During your exploration, you may find some key artifacts: Seven cities of gold gives you 200 golds in ancient and 250 in medieval, which comes very handy for your unit rushing. If you find Angkor Wat (and there is no English in the game), you'll likely get the Great Pyramid, then you can switch to fundamentalism making your attack force even stronger. The Knight from the Templar is a nice extension to your attack force, also giving a 9+6=15 attack with the vet galleon fleet.
Defence against rushers:
This strategy may seem very vulnerable to rusher attacks, since it takes a long time to get together that Armada and legions. However, the fact is that Madrid is kept on production almost all the time banking hammers for galleons and barrack. This means, if a rusher pops up, you can simply switch production to warriors (for early rush) or archers, or later legions and rush a sizeable force instantly. Your militia in Madrid is able to give you enough warning for both the horses and impis, unless they have a march upgrade or the opponent is a Mongol in medieval (you should place out some units for early warning in that case). You should also not spend all your gold on barrack/galleon/settler rushing at any point, always keep some in the bank for emergency defence rush. Once you get BW, you should also make an archer in Madrid to be safe against any surprises, but do not waste hammer/gold on making an army that early. Once your attack force is out, then you can build one.
Expand and tech further:
Even though, you are not likely to lose your fleet and the legions are cheap to replace and continue attacking, you should plan for a longer game, because your opponents will get pikes/cats soon enough and that severely limits the effectiveness of your Armada. Although you can also switch to cats instead of legions to keep up the pressure a little longer. Please, refer to the fine Spanish strategies in the archive for the further planning. Basically, you want Code of Laws for expansion, Currency (if you can get it first), Monarchy for the dye, Construction, Engineering, Invention, Steam and Democracy once you finished harassing others.