Over the course of time, many of these stelae fell over due to several reasons: structural collapse (as, probably, in the case of the Great Stele, measuring 33 m), possibly immediately after their erection; earthquakes (Axum is in a seismic zone); or the military incursions of the Imam Ahmad Gragn during the Ethiopian-Adal War from 1529 to 1543. In the 19th century, of the three major "royal" stelae, only King Ezana's Stele remained erect, shown in the print "The Obelis of Axum" of Henry Salt (1780–1827). Salt travelled back to England with Captain Thomas Fremantle, and the design of the Obelisk of Axum influenced that of the Nelson Monument, Portsdown Hill, for which Fremantle raised the funds.