public int TileID
{
    get { return BaseTiles.Count > 0 ? BaseTiles[0] : 0; }
    set
    {
        if (BaseTiles.Count > 0)
            BaseTiles[0] = value;
        else
            AddBaseTile(value);
    }
}

when setting we make sure there is at least one tile in the BaseTiles list and set it to the passed value. If there isn't, we create one with the passed value. Of course, we will need to add the AddBaseTile helper function to our MapCell class:

public void AddBaseTile(int tileID)
{
    BaseTiles.Add(tileID);
}