Sounds like you outplayed him to me.
I don't think staying at 4 techs forever w/Spanish is always the strongest approach unless you have no choice. If someone is going to do this, then you might as well just go Alph->Writing->CoL so you are only losing 1 pop per settler & can build all your settlers fairly quickly as the cities are keeping more pop & thus producing gold more quickly. I think building all your settlers at 4 techs is good but once you've got around 6 cities built & another 4-6 settlers on their way, it's time to start moving up the tech tree towards Democracy. If you're <=3 turns to 5 techs (should be a no-brainer), just wait it out & get all 3-pop in all your new cities. The Spanish can build settlers in Democracy just fine due to the same reason 2-pop cities are ok: the whale. Spanish can win w/6-15 cities. 6 will be challenging, 15 should be essentially a lock, 10 is just fine. Any more than 15 is simply overkill & not required. Also winning currency is quite helpful for Spanish usually which is usually easy to get first if you just let the other guy go for Irrigation (highly likely) & watch the tech tree closely. When they get Writing, just get Currency or if you like playing it more risky, when they get CoL, go get Currency. By this time, you should have plenty of settlers & can now start teching up to Democracy. Not getting Oxford is a huge mistake & if he built all these settlers, he could have easily been in Democracy at 13 techs, saved a ton of gold & rolled all over you later on if he planned properly & had built a few markets around & then went for Oxford after popping the 14th tech (banking) w/at least 103 tech/turn. Rushing libraries early in Ancient when your pop isn't 4+ is not a great idea IMO. However, if you've got 4+, it can be effective.
W/regards to Oxford usage, I like to use my wing for "softening" & then just follow-up w/land units or single units. I hate losing bombers as it's like losing a cat army so I prefer to strike w/vet wing first & then follow w/vet single unit, although a vet single is 27 attack which is like vet Jap knights so pretty darn good. I'll keep cranking out singles as needed to begin attacking 3 cities simultaneously. In almost all cases, it's highly unlikely the city has more than 2 defensive armies. If it does, I just shift to another city using 6 movement of bombers AFTER I see the flag go up (they just rushed more units) since it's even more unlikely they have 2 defensive armies in 2 cities. Now I go through the entire empire like that & in the meantime, I'm moving boats & land units (horsemen) for taking the empty cities. So just like in jet fighter sims, it's more efficient to do a "sweep" through an area & then turn around for the "return sweep" than constantly running in circles. By doing this & using the "wait till they rush it tactic", you can attack & destroy the defenses in multiple cities over 24 squares of movement in 4 turns & prepare the way for taking all those cities all the while burning your opponent's gold reserves & keeping him from teching due to paranoia about any city across his empire. Early bombers are really tough to counter when setup properly in my experience. You can usually take at least 3 cities in 6 turns. This usually means they have to already have Combustion in order to get flight in 6 turns or less. Even in that case, you'll likely still get 3 free cities & now you can defend your territory w/bombers easily.
Ok, I got carried away but I just love bombers!

For such an easy tactic, I've never had them used on me. I just find that strange. Once I figured this one out, geez, I use it anytime I can. It's just that strong. Given I posted the "recipe" on the forums months ago last year, even more strange.
best regards,
Pedal2Metal