As gorba said, FPSC draws a big collision box around dynamic entities, you can't have polygon collision for anything but static. If you look at the scripts for doors for example, you will see that they turn collision off after the animation.
A solution to your problem would be to produce 3 entities.
1. A tower, which is static and does move.
2. The large object, animated to move from upright to horizontal.
3. A large object of similar shape to the animated one, but horizontal and not animated.
Then, you just need to script the animation to run, and put "activateifused=1" in the script after the animation call.
Make sure the non-animated shape is set to dynamic, immobile, spawn at start = no, and spawn after delay = yes.
Then you shouild be good to go
Best,
Paul
