Getting varaibles from objects of different types inside an object array

Create an interface and an array of objects that implement that interface:

// choose a better name and make the properties read-only (get;} if necessary
public interface ILevelItem  
{
    public int XPos {get; set;}
    public int YPos {get; set;}
    public SolidBrush Brush {get; set;}
}

public class Wall : ILevelItem
{
    ....
}

public class Obstacle : ILevelItem
{
    ....
}

Now your "level grid" can be an array of the items

public ILevelItem[,] levelGrid;

and you can access the properties without casting:

int x = levelGrid[0,0].XPos;

You should decorate your different objects with interfaces, i.e. IObstacle, IWall, IMonster, ITreasure... then you can easily test if objetsOfWorld[i] is an obstacle, or a wall.

Then you can access the needed property (x, y, brush).

If theses properties are shared by all objects, you may have a parent interface, IObjectOfMyWorld that exposes the 3 properties. Then your array will be an array of IObjectOfMyWorld.

I think you can't use "object" here, because it's the type of every object you create in your application. You should create an abstract class with all these properties (xpos etc.) or an interface. All other classes which uses these properties should then implement this interface or extend the abstract class.

abstract class YourAbstractClass
{
    public int XPos { get; set; } 
}

class WallObject : YourAbstractClass
{

}

Then you can have one List<YourAbstractClass> with all your objects.

You should search for the keyword inheritance ;)