Was the titan in the last episode of AOT S1 inside of the wall or on the other side? Also about walls origin
Please try and phrase your question so that it is not ambiguous. 

Also, explaining this will be a major spoiler. 

However, a few answers can be given. 


   1) Yes, that titan is inside the walls. 
 
 2) Yes, that titan created part of the wall with its hardened skin
 
 3) No, it is not trapped.
 
 4) The colossal titan can destroy a part of it, but not the whole wall.