Loading game objects dinamycaly

First of all, look into XML serialisation, I find it easiest to serialise directly to a class. For example in my simple 2D game I use this class

 public enum Map
 {
     Alps,
     Canyon,
     Jungle,
     City,
     Space
 }
 
 public enum Tile
 {
     Standard,
     Sliding,
     Breakable,
     Broken,
     Worm,
 }
 
 [System.Serializable]
 public class LevelData
 {
     public Map MapName { get; private set; }
 
     public Tile[] LeftWall { get; private set; }
     public Tile[] RightWall { get; private set; }
 
     public LevelData()
     {
         MapName = Map.Alps;
 
         LeftWall = new Tile[8];
         RightWall = new Tile[8];
     }
 }

With Unity it's best to use Resources.Load to load your xml as then it will get bundled into the final build. I ran into issues with this because I'm building on mobile devices.

 LevelData data = null;
 
 TextAsset text = Resources.Load(path, typeof(TextAsset)) as TextAsset;
 
 // set up streams to read text asset
 XmlSerializer serialiser = new XmlSerializer(typeof(T));
 using (StringReader stringReader = new StringReader(text.ToString()))
 {
     using (XmlTextReader xmlReader = new XmlTextReader(stringReader))
     {
         // deserialise object
         data = serialiser.Deserialize(xmlReader) as LevelData;
 
         // close readers
         xmlReader.Close();
         stringReader.Close();
     }
 }

This allows me to have an XML file that is easily readable and modifiable.
These files must be saved as .txt to be read in by Resources.Load however.
As whydoidoit mentioned, it's best to only do this at level start up.
From this data, I can iterate through each tile and use GameObject.Instantiate() to instantiate prefabs at given locations. Personally I have the tiles already set up and then I simply iterate through and change the GUITexture. But the system is similar.

Hope that helps!