What kind of prevention measures are in place?

You've got the tower staff and a police squad, plus the fact that you cannot climb very quickly on those huge iron girders so by the time you're three meters up the ground you have twenty people around you. They won't try to wrestle you down if you've managed to go high enough, too dangerous, but they will funnel you to where you can be plucked and taken away. They do have climbing gear at the Eiffel Tower and have trained intervention teams.

But try to do it when it we are not on red alert, having guns pointed at you is so anti-fun. Then you have the fun part to explain:

1- you are not a terrorist

2- why you thought it would be funny to have the Eiffel Tower closed and evacuated in case you were a terrorist

3 - why you thought it would be fun

4 - why you thought French police had a sense of humour

5 - why you thought being a foreigner would protect you against French law

As for the penalty for trespassing and breaking law, I have no idea. But it won't be cheap.