I'm making a classic RPG and I need to be able to tell if the player is in a desert, plain, forest, etc, in the same map so I can spawn the matching monster to fight.

The same thing would be use to make splash sounds when walking on water or cover the player's entity with grass up to the knee when walking in tall grass.

What would be the best way to know on what type of environment the player is standing?

Is it to put zones in every tiles of the map, linked to different effects?
In a map 200x200, that makes lots of zones. I think there was, a long time ago, a limit of 255 zones per map. Or might have been different zones. Might also have been Verge 1. Well, that limit is probably gone, but still, it's tedious.

Arrays of tiles? From (1,1) to (12,17) it's grass, etc. But everything has to be square or I fear it'll take 3 megabytes of data.

What's the best way to accomplish this?
Thanks

You can do any of the following:
1) Use zones to mark every spot in the area. But then you can't have any other zones on different terrains.
2) Use zones to mark the outer perimeter of each type of terrain. Better, since you can still have other event zones inside these regions. So for instance, you have a zone for shallow water, and another zone around that denoting the beach surrounding the water.
3) Use GetTile and manually figure out what sort of terrain you're in. Most difficult and annoying.

I recommend option 2. It requires the least code to happen. You just need a variable tracking the player's current terrain. And switch the terrain type in the specific zone events.

In the case of splashing and whatnot, you could use trigger.onStep = "PlaySplashEffect";

Well it isn't REALLY that hard, but you'd need to hook in a custom HookRetraced loop. Which I try to avoid for small, small things. Though this isn't something you can always sidestep around.