I keep receiving a CS0103 error and a CS1061 error?

I tried compiling my code and I managed to knock down the compiler errors by half. I keep receiving these errors. Invader.cs(9,42): error CS1061: Type TreehouseDefense.Path' does not contain a definition forGetLocationAt' and no extension method GetLocationAt' of typeTreehouseD efense.Path' could be found. Are you missing an assembly reference?
Path.cs(3,11): (Location of the symbol related to previous error)
Level.cs(12,7): error CS0103: The name _invaders' does not exist in the current context Level.cs(17,31): error CS0103: The name_invaders' does not exist in the current context
Level.cs(23,32): error CS0103: The name _invaders' does not exist in the current context Level.cs(27,36): error CS0103: The name_invaders' does not exist in the current context
Compilation failed: 5 error(s), 1 warnings

using System; namespace TreehouseDefense { class Invader { private readonly Path _path; private int _pathStep = 0;

public MapLocation Location => _path.GetLocationAt(_pathStep);


public bool HasScored { get { return _pathStep >= _path.Length; } }
public int Health {get; private set; } = 2;
public bool IsNeutralized => Health <= 0;

public bool IsActive => !(IsNeutralized || HasScored);

public Invader(Path path) //Constructor
{
  _path = path; 
}

//ex. single line method
public void Move() => _pathStep +=1;

public virtual void DecreaseHealth(int factor) //virtual meaning 
{
  Health -= factor;
  System.Console.WriteLine("Shot at and hit an invader!");
}

} }

https://teamtreehouse.com/workspaces/40952082# Here is my workspace.