It was declared a National Monument under old NMC legislation on 6 April 1936. The monument was designed by Sytze Wierda who was the Zuid-Afrikaansche Republiek State Architect, and was built by WY Vietch. It was unveiled by President Paul Kruger on 16 December 1891.
On the 12th of April 1877, Sir Shepstone annexed the ZAR (Zuid Afrikaanze Republiek) without firing a shot, on behalf of England because of the discovery of gold. All negotiations with England to get the ZAR back and give the Boers self governance failed. Paul Kruger called a meeting with 6000-8000 "burgers" on 8th of December 1880 at Paardekraal ( Krugersdorp) to take action against the British to recover the ZAR. They also took stones and made a heap at this spot to ensure that they will always , at this space, remember the pact that they made to recover the Republic from the English.
This led to the battle of Majuba on the 27th of February 1881, with the Anglo Boer War where they succeeded in recovering the Republic. A Monument was built over these stones during the period April 1890-November 1890, this was done to commemorate Self Governance and later the Independence of RSA. In June 1901, Lord Roberts an English leader, instructed men to remove stones from Paardekraal, which they did, and threw into the Vaal River, below the Train Bridge. The stones are still in the River, and the Monument is still at the place where the stones were originally laid. Krugersdorp was founded in 1887 by one of the leaders, Marthinus Pretorius, and was named after President Paul Kruger.
References
http://www.sunstreak.co.za/a/20405/the-conquering-of-the-republic-of-south-africa--history-of-the-paardekraal-monument