Here's a rough anti-ezio matchup i drew up for a friend in PM's. its not perfect, but I think its a good beefy way to start off this discussion:
ok so the main goal of this matchup should be to zone ezio out, which will essentially allow you to avoid most of ezio's annoying tools(ex. 2K). this is perfectly doable, but remember that if he gains the life lead, he is capable of flipping the roles on you, and thats not the kind of situation you want to be in, so be careful.
alot of siegfrieds more punishable moves ie. 22BB, 66B, ect, ezio is incapable of punishing without a CE(watch ezio's meter carefully). you can use this to your advantage by using them on oki to zone him out with pushback, and to build guard pressure. being punished actually isnt an extremely large worry in this matchup.
ezio has a relatively decent oki game, and strong guard crush to boot. particular tools you may want to watch out for are 2A+B, 44B, ect, although he also has more standard options like 2K. after most knockdowns he is able to set up 44B block traps, which, if they can be rolled, can only be rolled to your left - although beware of 11_77B, which tracks oki to that side(though it can whiff if he is too close) and does a fair amount of damage, but has quite a bit of whiff recovery. both are reactable if you just lay there, but that doesnt mean you necissarily want to block them. 44B also does alot of damage on hit, so you dont want to give too many up for free. but basically unless he 2K's, ezio will have to commit to which side you roll.
ezio's 2K is a relatively nasty tool, which could potentially be used to pull out of clutch situations or even pressure siegfried in close range. here's a list of mids that it TC's:
basically, any mid that is vertical that covers ezio head to toe will hit him. anything that is mid that does not touch the ground, will not hit him. this also makes lows a more interesting choice against ezio.
the choice punish for ezio against siegfried lows is WR K, which has deceptively long range. however, if spaced out correctly, he will not even be able to punish completed low strings on block. that is not to say however, that the player wont just jump the second hit, but it gives us a little bit of free room to work with mixup wise if they arent reacting properly.
try to refrain from whiffing at range, as ezio can whiff punish you at any range using crossbow. this whiff punish also enables him to pretty much instantly close the gap. he can also kill backstep with crossbow, which can result in the same thing.
beware of stepping overly much against ezio, as he has a large selection of mid step kills to choose from - 22_88AA, 66A, 66AB(to make opponent freeze after 66A blocked), WR A, and 3A. a few notes about his 3A is that it is also only -2 on block, so he could potentially try to push offense after it is blocked. however most of his non horizontals are actually quite linear, especially to your left, so keep this in mind.
ezio doesnt have many good lows, but he has a good grab game and strong grabs to use it with. that doesnt mean you should duck against him, but rather use TC's wisely, and zone him out so hes not normally in range to grab.
ezio isnt the safest character in the game - alot of his strong verticals and strong horizontals are actually quite punishable. practice these punishes and make sure to take advantage of his bad positioning. because ezio is so punishable, players may stop mid string to try to keep the pressure going through fear advantage. as with siegfried, always be aware of this and try to go out of your way to stop it as much as possible, unless you want to get run over.
ezio also has a fairly decent evasion game. beware of 4B, 4B+K, 2K and the like as they all have great evasive properties that make hitting him harder. as with lexia, cervy, rugi and the like, you either want to zone him out so that these arent very effective, or you want to deal with them ASAP in a fitting manner.
as mentioned before ezio has a very nasty guard crush game that is relatively easy to access. here is a list of moves that crush in under 15, and some basic frame data to go with them:
6ABB i17, -16 on grd, crushes in 8.
4AB i21, -30 on grd, crushes in 9. - this is actually a CH stepkiller, so pay no mind to it you probably wont ever see this on block.
4B i34 -18 on grd, crushes in 8. - this is just an evade, but im certain because its -18 on block and launch punishable, you will probably be blocking this alot when you're not stepping or whiff punishing it. a guard crush is realistically possible off this move.
WR B i18 -12 on grd crushes in 14.
WR BB i18 -16 on grd crushes in 7. - its a relitively slow string to come out. try seeing if you can JG this, because its not stepable.
4K i18 -8 on grd, +4 on hit, +12 on CH crushes in 14. - this kick is a tad slow, but like b6, it has alot of range, and is a decent enough poke. the crush damage isnt too threatening, but do be aware of it.
A+B i25 -16 on grd crushes in 15. - this move can also transition into crossbow evade, so if you try to b6 punish he can at least get a side grab or a back grab.
2A+B i24 -6 on grd crushes in 11. - good oki move, you will probably see this alot.
6B+K(standing crossbow) i21, -30 on grd crushes in 11. probably not something that will crush you. again this is more likely to kill your backstep, and if it does crush you, it will pretty much have the same effect as if it hits you - ezio gets in.
2B+K(low crossbow) i31, see above.
4B+K(evading crossbow) i31, -30 on grd, crushes in 10 - this could actually realistically crush you simply because it is an evade move, which means you'll likely see it more often.
66AB(series) i22 -30 on grd crushes in 9. - if he makes bad read and you turtle it out this could potentially crush you, but i wouldnt count on it.
22_88AA - i35, -23 on grd(first hit is -15) crushes in 12.5(15) - obviously ezio will use this move alot for containing movement, but because it has fear advantage and doubles it up with decent crush damage, this move could crush you; although more often than not your penalty would probably be eating the rest of this string.
11_77A i41 -20 on block, crushes in 15. - this again, wont be used as a crush move, but you may begin to see it if you frequently kill ezios step with lows, as it TJ's out of step.
66BB i20 -16 on block crushes in 8. - this you will probably see alot, along with its BE countarpart. try JGing or even GIing it. you can also step it to his left and get a punish as its relatively whiff unfriendly, and it is also the safest option.
22_88B i31 -12 on block, crushes in 11. - this move crushes alright but has limited range and is pretty linear to boot. try taking advantage if that.
11_77B i49, +8 on grd, crushes in 6. - very dangerous oki move because of how much KND advantage ezio gets. tracks roll to your left, making this a force block or even a nasty hit. too slow otherwise(JGable, GIable and stepable on reaction.
44B i39 -10 on grd crushes in 8. - very very VERY dangerous move on oki you will see it used frequently i guarantee it. it is a forced block or roll trap after most KNDs, being only escapable to your left. very easy to land, very easy to crush with, very safe vs sieg.
22_88K i32 -6 on grd crushes in 11. - could be easily and safely landed for a guard crush, and gives oki on hit besides.
44K i24 -12 on grd crushes in 10. - could be used as a backstep killer when sieg is at disadvantage BUT range is not as good as it visually appears. could crush as a result of this purpose, but could also just be thrown out as a safe guard crush.
66A+B i31 +10 on hit crushes in 9. - easily duckable on reaction but could be mistaken for 11_77B in clutch situations, hence carry half a bar of meter for emergency GI's.
BbA+B+K i16 -6 on grd crushes in 8. - this will normally be only ever seen as a punish, and is easily stepable and reactable as a string.
236236A+B+K i13 -18 on grd crushes in 11. yes ezio CE can crush you.
now ezio is a bit slower than other characters in his class, but he has the advantage of being able to muscle things out. that doesnt mean he isnt still able to do cheese such as 2K 2A, but its a bit more difficult for him to maintain constant lockdown and pressure. while this means siegfried can feel a little bit freer to move, where his fear comes from is fear advantage and crush game, so keep this in mind while fighting.
look out for the move 6k2. its a NH 36 damage low, but it is also WR B punishable even on hit(on block its -32, so insert move here) it is a good round finisher because of the amount of damage it does, so beware when you are in the yellow.
thats all i've got for now. ill add more later if i can think of more later.