if (choice == 1) {// choice 1 (or 0 in the term) is leave.
message_dialog("blah,","");
end();
}
flip_terrain(?,?);
break;

is less time consuming then

if (choice == 1) {// choice 1 (or 0 in the term) is leave.
message_dialog("blah,","");
end();
}
if (choice == 2) {
your codes here.

beginstate 16;
check_text_response_match("imdone");
text = got_text_match();
if (text == 0) {
set_state_continue(19);
end();
}
if (get_terrain(6,41) == 268) {
message_dialog("The panal glows. _The portal of command imdone is now closed_","");
set_terrain(6,41,0);
end();
}
message_dialog("The panal glows._Ok. Portal Open. Please go to the top of the huge stairway near the south entrance","");
set_terrain(6,41,268);
break;