The Great fortress is surrounded by 5 miles of landmines. It has a large moat filled with strong acid and a drawbridge. It has 50 foot high concrete and Kevlar walls. The guards stationed on the walls are complete idiots, although they possess very powerful anti-aircraft, anti-tank, and anti-personnel weapons and will shoot anything on sight. Lastly, the castle has an anti-force field superweapon.
Also, heavily guarded supply trucks run in and out of the fortress daily, with photo id checks, fingerprint checks, and password checks.
It runs on its own electricity and water.
On the inside is $10,000,000, an invention that will make you famous, the love of your life, and a pet dragon that will remain loyal to you forever (you want to get in).