all depend on what your level contain....
However the file can be a xml or json file, that represent the x,y of where special objects are (like star, wall, etc...)
a short example to represent a level in xml with a "wallB" in 125,225 can be
Code:
<level>
<object>
<name>wallB</name>
<x>125</x>
<y>225</y>
</object>
</level>
You can create a level editor to make the file easily.....
To parse it you can use NSXMLParser....