Premade Background issue

Hello all im having a bit of trouble here, so im trying to test myself by attempting to receate Zombies Ate My Neighbors (old snes game/2d top down shooter). The reason i choose this challenge was mainly because im not that creative when it comes to game design and things like that so instead im using premade sprites and level backgrounds. Now here is my issue, so with the level 1 background it comes all togther without boarders or colliders just one photo or sprite if thats the best way to put it. How should i go about making certin things interactable when its all just on sheet (the level has doors/pools/trees/grass bushes that act as boarders). I figured i could put colliders and somethings like the bushes and trees so the character doesnt walk right threw them but then that still leaves me with the doors that swing open and also the pool that you jump into...any ideas on how i can get this done or is it impossible??

p.s. i do have the sprites that have the door open as it shows on the top left corner.. thank you to anyone that can help me out even in the slights

 

you have to separate the walls from the floor

you need to have the floor on one layer and the walls and trees on another layer

the door also won't work as you want unless you crop out the floor

basically if you want to walk behind the trees and behind walls you will need to create a tileset

 

ok thank you for your reply