I had to beat fake moon landings. And they can still cut through the fence.
The Maginot Line.
The 'West Wall' Defense.
The Siegfried Line.
The Berlin Wall.
Any fortification, any defense, will eventually be penetrated by a determined opponent.
Build it anyway, and then remove the incentive (perks and bennies for illegals).
Hit the problem from both ends.
I have cut steel on concrete, and it takes time. Whoever did that had time on their hands.