Kassadin top is a very different champion from Kassadin midlane. On midlane you do more damage due to the midlane quest, and you are squishier. You also play against squishier targets and you look to play like an assassin.

In toplane, you are way tankier due to toplane level 20 (extra hp, armor and mr), massive TP shield, you do less damage, and you scale better.

Gameplan and win condition
You survive lane, you get your core items, then you go to sidelane. You farm and take towers, and wait for big fights that you can join with your upgraded unleashed teleport which gives a massive shield. This is because it scales with your HP and you have a lot of bonus HP from RoA and HP runes. Once you TP in the fight you don't look for flanks like an assassin, you go straight in and engage the fight yourself, you are the toplaner tank. You have ROA, TP shield, Seraphs shield, Frozen Heart, your passive and your Q to protect you from enemy burst. Once you go in, the enemy team has to respond to you, and then you make advantage of your low CD ult. Either ult out if they all commit to you, or keep going in if the fight looks good and your team is following up. This is how you play fights. You constantly R in and out, because you are tanky enough to do that. It is very hard for enemies to lock you down when you R in when you have 5k effective health, 200 armor and 1 second R cd. They cannot ignore you either, as soon as you have malignance, you kill their ADC very fast if they try ignoring you, and your target access is very good as well, so it's hard for their support to stop you. So, to summarize,

1. YOU engage fights unless you have better engage on your team. GO in and tank for your team.
2. ULT out instantly if you have to after going in depending on how much enemy team commits on you, do not die. Even if you don't kill anyone or don't do any damage, just forcing the enemy team to commit to you is good enough most of the time.
3. Once you get level 20 and full build you become complete raid boss and you can do anything you want. You will both be tanky and have damage.

Give up some CS. You need to judge yourself when you can trade some of your HP for a couple of minions. If enemies can dive you, don't do it, preserve HP.
Keep wave close to your turret, wait for jungle ganks. If they don't happens, don't panic, just stay AFK and keep farming, you are not under any pressure, your enemy is. They need to kill you and snowball to win the game. Your job is to not allow that.

Poke enemies just enough with DFT / scorch so that they are never above 70% hp. This makes it much harder for them to dive, and you open up more gank opporunities for your jungler. Don't try anything crazy in 1v1s yourself.

If you are stronger - just go to sidelane and push.
If enemy is a tank you cant 1v1 like Mundo or Chogath, ignore them and take CS, proxy, ult over walls if they try chasing you, be very annoying. You have good waveclear with axiom arcanist, E and R. Just perma rotate to your team after you push waves.
If you are very weak - Don't side. If enemy top is grouping, sure go and farm sidelane, but if enemy is splititng, just group with your team and hope for the best.

You want to look for plays whenever your upgraded unleashed teleport is up, play around that shield, do not let it expire. Engage fights instantly after you TP.

Also look for fights when you got push on sidelane by proxying for example, and obviously for all drakes, if constestable.

Ult in until a lot of enemies click on you, then ult out. Fight in tight spaces, avoid araming mid unless you are stronger than the enemy team (your entire team, not just you). If you are ARAMing mid, the most important member is your/enemy ADC. They excel at those fights.

Almost anyone in the game once you are 16 with 4-5 items.
If there are champions that you cant straight up stat check (Nasus, Jax, Gwen, Renekton), you can at least hard engage on them and force them to ult. You can do this before important objective fights. Then just run away, maybe recall and be there for the objective. Boom enemy top is now useless while you still have all your cooldowns unless they got your seraphs.

If lane went bad and you are 0/5, chill. You hopefully soaked up some pressure from enemy jungler, so your botlane should be slightly ahead if your jungler crossmaps correctly. Give up a drake or two, try to communicate that with your team, and sidelane whenever the enemy top is grouping, never group with your team for a fair 5v5 fight.