The Spanish are primarily an offensive civilization featuring a strong Imperial Age army and navy. Most of their military bonuses can only be used in late games and are the most powerful after reaching the Imperial Age. Despite their emphasis of gunpowder units like their Turkish counterparts who would make an effective ally, they offer a wide range of technologies and are able to field a varied army.
Strengths
Conquistadors are essentially Hand Cannoneers on horseback.
They can research all naval technologies, which makes them formidable in the water. However their biggest naval advantage is that their Cannon Galleons benefit from Ballistics. This allows them to fire much faster and more accurately. In addition to performing their regular duties more efficiently this allows them to effectively engage warships and no longer need to be escorted by Galleons or Fast Fire Ships.
The Spanish unique unit is the Conquistador, a mounted Hand Cannoneer. Like the regular Hand Cannoneer, they have a large attack but are inaccurate. They have the advantage of being mounted which gives them an edge in speed, allowing them to use guerilla tactics. They are somewhat fragile so are best used as support units in formations with Knights or Paladins whose higher hit points can absorb damage that would easily kill a Conquistador. The Spanish also have the Missionary, which is a mounted monk with greater speed but have lower range and cannot pick up relics. Since Missionaries move much faster than normal monks and a Spanish army will be mostly composed of cavalry, this is a big advantage. The Spanish unique technology, Supremacy, gives villagers a large increase in attack, hit points, and armor. This allows them to go in the front lines and build military buildings to reinforce and support a Spanish army without getting killed as easily. While their Villagers are capable of defending themselves against certain units, they should not be strictly used for combat.
Weaknesses
Despite their large number of advantages in the Imperial Age, the Spanish have some drawbacks in the early stages of the game. They have poor foot archers, unable to improve their Archers beyond the Crossbowman upgrade. This puts their infantry units at a disadvantage until heavy cavalry can be trained. Their siege weapons are merely average, missing the Siege Onager and Heavy Scorpion.
Strategy changes in The Forgotten
Spanish Monks are greatly improved, as they already have many advantages that increases further because of their Castle Age unique tech Inquisition that makes the conversion rate of Monks and Missionaries more efficient. Missionaries also are now affected by Bloodlines making them more resistant to attacks enabling them to make a better frontline support.