This seems like a theme whenever I start on the same continent with Siam. I steamroll a city or two of theirs, and then they take it back with Naresuan Elephants. Or they just roll over every other unit I have in the field with them.

Swarms of Naresuans.

How to counter this crazy elephant UU of theirs when in Medieval/Early Renaissance era?

They still count as Cavalry, so the obvious answer is Pikemen. But Crossbowman work of course too (if you don't let your units get attacked). Or a mix of both ;)

Renaissance-era tercios would be the answer.

Yes, pikes and crossbows are the generic answer. But camel archers or keshiks are just as powerful against Naresuan's elephants as they are against anything else that only has two movement points.

If you're playing Germany, then spam Landstraat units, they are very good against Elephants, and cheap too...

Crosbowman+ pikeman Don't build longswordsman

They are slow and that's their weakness. I hate to face them in defense. But if you're skilled enough tactics-wise you can bait them into unfavorable position, outflank and kill. With some loses. Bring workers, scouts or whatever useless cheap units you have to use them as a bait. And you do need LS to take cities anyways. BTW, elephants aren't strong against cities.

They still count as Cavalry, so the obvious answer is Pikemen. But Crossbowman work of course too (if you don't let your units get attacked). Or a mix of both ;)

Yeah, I really should use more ranged units in my games that aren't gunpowder based.:lol:

They are slow and that's their weakness. I hate to face them in defense. But if you're skilled enough tactics-wise you can bait them into unfavorable position, outflank and kill. With some loses. Bring workers, scouts or whatever useless cheap units you have to use them as a bait. And you do need LS to take cities anyways. BTW, elephants aren't strong against cities.

I guess it depends on where Siam's cities are located - last game they were blocked by city states and a lot of hilly terrain. Getting there, much less getting reinforcements there, turned out to be an insurmountable problem. I find the AIs in general don't like to come out and play much after they see you getting advanced units. Sure they'll come back when they have their UUs and whatnot, but even then they'll retreat the moment they figure they're losing and then spam out more units before having another go.

And yeah, Elephants are weak against cities and ranged/gunpowder. In recent game as Siam, Rome managed to trash my force of Naresuans with cannons and crossbows positioned around their third city. That means I'll just have to spam crossbowmen from now on when going up against Siam.:lol:

I had Rammy try this the other day when I was China and had built the Great Wall (I only ever build it when I'm China). Chu-Ko-Nu's shredded like 10 elephants before they could attack my city more than once. Focused fire is great against the AI.

I guess it depends on where Siam's cities are located - last game they were blocked by city states and a lot of hilly terrain. Getting there, much less getting reinforcements there, turned out to be an insurmountable problem. I find the AIs in general don't like to come out and play much after they see you getting advanced units. Sure they'll come back when they have their UUs and whatnot, but even then they'll retreat the moment they figure they're losing and then spam out more units before having another go.
Nah, they retreat for one turn, move their units around pointlessly while suffering even more loses and then move forward again. When you face tough defense patience usually pays off. Wait until they come towards you and then kill them on your own terms.

And yeah, Elephants are weak against cities and ranged/gunpowder. In recent game as Siam, Rome managed to trash my force of Naresuans with cannons and crossbows positioned around their third city. That means I'll just have to spam crossbowmen from now on when going up against Siam.:lol:
Not against gunpowder, just cities. And everything in the game is weak against ranged. You cannot get wrong with crossbows no matter who you're fighting.

But let's summarize all of it this way: playing as Siam is much nicer than playing against Siam. :D