Bonatus describes a besieged planet as one that separates from a malefic and applies to another (Con. 6). It shows a situation that is going from bad to worse.
William Lilly described a planet besieged as one that lies between the bodies of the two malefics Mars and Saturn. His example is Mars at 10 ° Aries, Venus 13° Aries, and Saturn 15° Aries. It is generally accepted that besiegement can occur by aspect as well as bodily conjunction. As a principle it refers to a planet trapped by hostile forces on either side.
The diagram shows William Lilly’s description of Venus besieged between the bodies of the two malefics: Mars and Saturn (CA, p.114).