ATP wasn't always a thing, you know... If you're bored with the current meta, then certainly familiarise yourself with this guide!
As a civ that's been overpowered for most of AoE3's history, you could call it justice that Spain got nerfed into the ground. I can't help but disagree with the way they were nerfed though, as nerfing the shipments of a civ whose unique bonus is shipments seems like going against your own design. ES chose to do this anyway, and the result is absolutely abysmal unit shipments in fortress. 3 lancers? That's as valuable as 3 huss in colonial, and that shipment is rarely sent in colonial let alone in fortress. 7 rods? That's 700 resources, most of which food. I wonder who made these decisions...
Rants about design flaws aside, Spain is happy with the evolving crate meta. As it turns out, Spain scales extremely well with crate shipments since those shipments weren't nerfed into the ground and Spain does still get their shipments faster. All nice and dandy, you'd think. However, Spain lacks 1000g which is kind of a huge deal because now, since Spain is already missing RI shipments, 1000w and 2 falc are the only shipments you're excited about sending as you age up to fortress. And the poor unit shipments aren't Spain's only problem: their politicians, both to colonial and to fortress, are no good at all. This is a problem Brits face as well, but Brit doesn't really care whereas Spain relies on their (semi-)FF builds, which are a lot weaker without the fast age option. Especially semi-FFs are hurt by this handicap, which is very significant in match ups where their opponents are likely to semi-FF. After all, Spain can't really deal with that effectively; straight FF is countered by it, and prolonged colonial is hopeless. In semi-FF wars, Spain loses out.
Spain is a lot less happy with the evolving TP meta. They don't really need TPs (more than 1 is already overkill with this civ), and other civs spamming them all over the place kind of negates their civ bonus. They still get shipments faster and they don't have to invest in TPs early on to get this going, but it's still awkward to see your opponent building 2 TPs and knowing your civ bonus is basically irrelevant from there.
But they're not a weak civ. In fact, I'd say Spain has some undiscovered potential especially in an eco-heavy metagame, where their fortress options without unit cards will need to be explored and probably found surprisingly strong.
Spain ages up with a decent eco, comparable to (although admittedly somewhat worse than) the French and German one, and they get shipments faster.
The 500f instead of 400w is somewhat awkward, but manageable. Your quick 3 shipment spam in early colonial gives you a crazy number of options which makes it hard to write a build for Spain, but does make them a versatile civ. As mentioned though, they lack the option to adapt into a quick age up, which limits their options significantly.
Spain is a civ that likes to get at least something out of colonial before going up, or going up immediately. Defensive semi-FFs are not preferable with this civ as you will likely get countered by a faster semi-FF or solid colonial pressure. Spain is better off threatening pressure or even doing a timing to force their opponent to cancel their age up plans. Even though Spain is a good fortress civ with their infamous rod+lancer composition, the lack of a fast age politician makes their semi-FF so awkward that they would rather play a prolonged colonial in most match ups. This is what made them so strong on FP even though they weren't buffed all that much: Prolonged colonial wars were popular there, and Spain thrives at that. Their eco is actually great, and the abundance of shipments even allows them to squeeze in 4v, which no other civ would ever send.
I've personally been playing around with the idea of sending 1 falc in fortress with this civ. After all, to make a falc you'd have to spend 300w on the art foundry and another 500 resources on the falc itself, and generally you won't want more than 3. 3 falcs allows you to win the artillery war against civs with a 2 falc shipment, and falcs are great against colonial compositions as well. Because Spain likes to FF in some match ups, they will often face colonial compositions, making this shipment a viable option.
Yes, 800 resources is not great for a fortress shipment (though 400 of that is wood, which you won't have market upgrades for), but look at all the other shipments that Spain has available to them in fortress. Is there anything better? Besides 1000w and 2 falc, there isn't really. Largely untested, but good in theory. I suggest you put it in your decks and see where it lands you, after all there's plenty of space in Spain's deck because of all the useless unit shipments.
The Spanish build
With 200w, build an early TP and chop for the house. With 100w+100c, go early market (market first, trade coin for wood). With just 100w, simply build a house. This is likely to get you a fast age up and the early market may get you a late age up. Not worth the risk in my experience.
At this point you should already know if you're going for the FF or a colonial start, because here is where your build starts to deviate. FF is strong against civs that are not comfortable semi-FFing or have a slow, punishable (by your no eco early fortress push) semi-FF. Obviously it's also important that they don't have the option of simply rushing you to death.
The FF 30%
Spain's is by far the most versatile FF. They have many ways of FFing, all viable in different situations. All the variations can be grouped into 2 styles of play though, being the 5v FF and the 700g FF. There's also the anti-rush tower FF:
The Tower FF:
Age up with the tower and 200c, make sure you gather enough for minutemen as well as the age up (so 1350-1150 instead of 1200-1000) because you're up against a rush (if you're not up against a rush you shouldn't be doing this kind of FF). Chop for a transition TP.
(3) 700w or (3) 8 pikes if you desperately need units to hold or want to turn this into an early fortress timing.
Gather food for dogs while you age up, they can help hold.
The 700g FF:
Age up with 500f, chop for a TP. If you already have a market, get placer mines and chop/mine enough for early steel traps. If you don't have a market yet, chopping for it is viable but only if time is not that much of an issue. Your FF will be slightly slower but obviously stronger in the long run. Generally the market is better unless you're going for an early fortress timing.
From here this FF has some options, which makes it able to adapt and therefore a solid choice:
(3) 700w for a solid early to mid fortress mass. Build a rax from this and get a market going if you didn't have one already.
(3) 5v for a long term fortress play. Your first card in fortress in this case is always 1000w. If you can't afford to send that because you need units, 5v was a mistake.
Chop wood for a rax/stable and a house or 2 so you can make at least a couple of batches in early fortress while you wait for 1000w. Skip the market until 1000w.
(3) 8 pike for an early fortress all in push. Your next shipment is generally 2 falc and then for continued pressure any combination of 12 pike, 4 lancer and 8 rod (in order of strength). To transition out of this or take a break from the aggression, ship 1000w.
The 5v FF:
Note that this FF is slower, but obviously eco-heavy.
Age up with 500f, chop for a TP. Skip the market for now, unless you already had it in which case you should get placer mines.
Set up your vills to gather the full 1200-1000 without resource shipments.
(3) 700w. Market from this (steel traps and amalgamation), as well as a military building.
(4) 1000w for houses and a second military building.
The aggressive start 40%
Even though Spain doesn't have a 400w politician, they still have some decent aggressive options in colonial. This involves starting 2 military buildings and sending 700w for houses, much like France or Germans. The difference is that Spain has to chop the 400w, which is a big deal but not game-breaking.
Age up with 500f. Chop wood for a TP. The market (if you didn't have it already) is optional, you can also choose to delay this until 700w for a better mass in early colonial. Chop 400 (optional: 525 for steel traps) more wood for 2 buildings and move vills to gold/food. Keep in mind you're getting 500f from age up.
Build either 2 raxes or rax+stable as you hit colonial. Note that you have the advantage of not having to gather the 400w before starting the military buildings, but keep in mind you'll be housed if you don't get a house up in time. For this reason you should send 700w asap.
(2) 700w. Use this for houses and optionally for remaining market upgrades.
(3) 700g for any combination of musk, huss and rods or (3) 600w for xbow+pike.
Note that (3) 5v is also viable to transition out of military-focused play and go into a long game. Refer to the long term plan for further options.
(4) 600w or (4) 8 pike for a timing or (4) 600/700g (age up optional).
The long term plan 30%
Spain can also choose to start somewhat more passively and make their build depend on their opponent. Spain has the advantage of getting more shipments, so they can squeeze 5v in with all the crate shipments. If you're going to do this, it's always best to do it asap, but there's still 2 options: 700w first or 5v first.
Age with 500f. Chop for a TP and a market. Make sure you have enough for steel traps and enough for a military building as you hit colonial. You'll also need to keep chopping wood for a house or 2 in early colonial. Usually you'll build a stable, because that way you'll have the option of raiding. A musk start is also good and preferable in some match ups because it's easier to age up behind.
Doing this the other way around is also viable. This allows you to transition into a somewhat stronger and less cav-dependent (if you started stable, that is) mid colonial mass by mixing in 2 more military buildings from the 700w.
(4) 700g (age up optional). (4) 600w is also viable but it's advisable to stick with 700g simply because it gives you the option to age. Spain ages to fortress slowly, so they can't really make the snap decision to age up. You have to keep a potential age up in mind during the early stages of colonial by sending gold shipments and keeping your options open that way.
(5) 600w if you decided to age up, (5) 600g if you didn't. 600w may still be better in prolonged colonial, but again you want that option to age up.