This was a fun challenge. I am new to the game, so I needed some attempts, but finally did it. Btw, did it on my Mac, which is quite tough sometimes.
Lost 1 civilian, because I fed my knights and sword men, argh.
Also, couldn't you easily cheat by loading saves all the time? Therefore, I would suggest different levels of achievement:
20 bowmen/20 swordmen/5 horses/ 3 towers up/ 20 civilians lost --> Bronze Level
30 bowmen/30 swordmen/7 horses/ 4 towers up/ 10 civilians lost --> Silver Level
35 bowmen/35 swordmen/10 horses/ 5 towers up/ 0 civilians lost --> Gold Level
I don't even know, if the last one is possible with the old build order, but maybe someone wants a challenge.