To anyone wondering: yes, the Shock tower gains damage from ANY guard tower in its vicinity, both basic and upgraded.Couple it with a couple Fire rate and range upgrades and you have the magical equivalent of a minigun that shoots explosive sharks.
GLITCH:If you use Healers, after you order them to heal you can attack the castle with a siege tower even if the front line isn't down yet.
Made castle fights a cinch.
PRO TIP:Idle to level 20 and increase only beg rate.Then beg, buy more points, keep increasing beg rate, buy shoes, hat, sign and clothes.Then buy violin and skills and only when you have so much money you earn it faster than you can spend it, max drinking rate.