How can I make a level with different environments?

Finally back to GS 
and lately I've been stuck having problems with making a unique level, I'm doing a side scroller and I'm wondering how can I have actors outside of the view safely without it being destroyed.
I've looked into timers but, I mean I can't just spawn a platform or wall in front of me after 15 seconds or so, I mean it works for enemies but, not the environment. Anyone got a clue or guide to this?
Andddd also, isn't spawning bad for the iPhone? I need a safe way to play it in my iPhone xP

and lately I've been stuck having problems with making a unique level, I'm doing a side scroller and I'm wondering how can I have actors outside of the view safely without it being destroyed.
I've looked into timers but, I mean I can't just spawn a platform or wall in front of me after 15 seconds or so, I mean it works for enemies but, not the environment. Anyone got a clue or guide to this?

Andddd also, isn't spawning bad for the iPhone? I need a safe way to play it in my iPhone xP
Comments
" mean I can't just spawn a platform or wall in front of me after 15 seconds or so, I mean it works for enemies but, not the environment. "
...You can spawn any actor with the right brain power.
:-)
and thanks for the replying