Using a grid for traversal creates simple and cheap collision checks for the player and enemies that dictates where objects can move and be interacted with.

The grid is a simple 2D grid with height data inside used for the y placement of objects aswell as pathfinding. This meant that while moving we'd check your cells height data and compare it to neighbouring cells to determine if you are able to walk there or not. For stairs we simply added a step height and check how far along you are in the stairs direction to decide your height. Elevator worked by increasing the cell's height data over time making objects in the cell move up or down. For obstacles it was a simple on or off switch that says if a cell is traversable or not.

To make it easier for level designers height data is calculated when a scene is loaded and grids are generated so level designers didn't have to  be super thoughtful when placing floors, stairs and other traversable tiles. They only had to make sure it's placed on coordinates with whole numbers, but setdressing and obstacles could be placed anyway they wanted and during scene loading everything will be applied and work correctly.

When accurate collision detection was needed such as during attacks and projectiles AABB collision detection was used. Objects only had to be checked against others within the same cells as objects could be within several cells at the same time depending on their collision boxes.

Enemy AI is split between the two enemy types, The melee enemy uses simple seek behaviour with A* pathfinding to chase and attack the player. The ranged enemy uses seek to get closer and attack but if the player gets to close it runs a simple flee behaviour