Access Control / Military Science of Fortification: 

Wire Obstacles are used to defend high-security buildings like government buildings. Walls and fences are more difficult to climb if a wire deterrent topping is applied.  They typically adorn the top of strong chain link fences, steel pickets, or stone walls.  At Buckingham Palace, they adorn the tops of high stone and brick walls. If someone tries to jump the fence, there is almost a 100% chance that they will get caught in the security wiring and injure themselves. Wire deterrents are meant to reinforce perimeter security and to slow down or stop an attacker. Barbed wire, barbed tape, and concertina wire are the three main mechanisms used in wire obstacles. Anti-climb fence spikes are also used at times for the same purpose.

Barbed Wire

Barbed wiring are steel ropes with sharp points or 'barbs'. Razor barbed wire is used in high security settings. The hostile appearance of barbed wire is sometimes enough to keep intruders at bay. However, wire cutters can cut through barbed wire and attackers are sometimes prepared to evade barbed wiring. 

Barbed Tape

Barbed tape, or razor wire, is different from barbed wire. Razor wiring looks like sharp razor blades built into a thin steel rope. Barbed wiring  looks like several small coils of sharp steel wrapped around a thin steel rope. Razor wire is sharper than barbed wire, but not as sharp as actual razor blades. Barbed tape is harder to circumvent than barbed wire, and thus is a more effective fortification mechanism.

Concertina Wire

Concertina Wire, also known as Dannert Wire, is large circular coils made out of thin ropes of barbed wire. They are coiled in conjunction with steel pickets or steaks. Constantine wiring can be set up quickly in a war setting, or are permanent fixtures commonly found along prison walls.

Factory made and used since World War I, Concertina wiring in a staple in modern fortification science.

 Anti Climb Fence Spikes

Security fence spikes are various types of systems constructed with a thick horizontal steel pole completely covered with very long and sharp angled steel spikes. They are designed to increase levels of security against intrusion, terrorism, vandalism, and to protect high-security buildings. Wire cutters cannot cut though this type of fencing.

SOME Security Breaches at Buckingham Palace:  [From dailymail.co.uk]

:: The most serious breach came in March 1982 when Michael Fagan broke into the Queen's bedroom at the palace. She woke to find him sitting on her bed.

:: In 1992, an intruder walked into St James's Palace and downed a whisky in Princess Alexandra's private apartment.

:: In 1995, student John Gillard rammed the palace gates in his car at 50mph, tearing one off its hinges.

:: In 2003, an undercover reporter from the Daily Mirror got a job as a footman at the palace.

:: The same year, a major investigation was launched after "comedy terrorist" Aaron Barschak gatecrashed Prince William's 21st birthday party at Windsor Castle. Wearing a dress, beard and sunglasses he climbed on stage as the prince addressed the crowd, and kissed him on both cheeks.