Sling as siege is just not good at all. If you want a unit good vs. towers then make it melee cavaly, because tower have minimum range (melee cavalry approach tower fast so not under fire for long, they get under tower's minimum range and are free to chop it down). later you have tower tech that remove minimum range so now melee cav arent so good vs towers, which okay because now you have siege catapults to take care of them. Tactic progress as match progress.