ad ella . ▁ ▁S light ly ▁less ▁than ▁three ▁years ▁later , ▁on ▁ 2 1 ▁April ▁ 1 5 5 5 , ▁after ▁more ▁than ▁a ▁year ▁under ▁sie ge ▁by ▁Spanish ▁and ▁T us can ▁troops , ▁the ▁city ▁surrender ed . ▁▁ ▁The ▁Republic ▁of ▁S ien a , ▁supported ▁by ▁its ▁ ▁French ▁al lies , ▁continued ▁to ▁def y ▁the ▁Spanish ▁from ▁Mont al c ino , ▁but ▁the ▁city ▁of ▁S ien a ▁itself ▁was ▁now ▁back ▁under ▁Spanish ▁control . ▁▁ ▁Four ▁years ▁later , ▁in ▁April ▁ 1 5 5 9 , ▁the ▁Peace ▁of ▁C ate au - C amb rés is ▁was ▁signed ▁between ▁France ▁and ▁Spain . ▁▁ ▁This ▁left ▁the ▁Republic ▁of ▁S ien a ▁without ▁al lies ▁and ▁led ▁to ▁total ▁surrender . ▁▁ ▁The ▁city ▁fell ▁under ▁the ▁control ▁of ▁Spain ’ s ▁regional ▁al ly , ▁the ▁Flor ent ine ▁Duke ▁Cos imo . ▁ ▁In ▁order ▁to ▁prevent ▁any ▁attempt ▁by ▁the ▁S ien ese ▁to ▁recover ▁their ▁independence , ▁Cos imo ▁ordered ▁the ▁construction ▁of ▁the ▁present ▁for tr ess ▁on ▁the ▁site ▁of ▁the ▁earlier ▁Spanish ▁Cit adel . ▁▁ ▁The ▁project ▁was ▁entr usted ▁to ▁Cos imo ’ s ▁celebrated ▁U rb ino ▁born ▁architect , ▁Bald ass ar re ▁L anci ▁who ▁had ▁started ▁his ▁career ▁as ▁a ▁military ▁engineer . ▁▁ ▁Const ruction ▁comm enced ▁in ▁ 1 5 6 1 ▁and ▁was ▁completed ▁just ▁two ▁years ▁later ▁in ▁ 1