The Town boss admits his side will be heavy underdogs against a side featuring two former Port Vale players and last season's 44-goal striker Damian Reeves.
But Casswell has not discounted his side's chances."We have been under pressure against sides in lower league to make sure we get through," said Casswell.
"This week we are almost in a no lose situation. The one thing we're saying is 'make sure you're not disappointed with your own performance when you come off the pitch'. If they all do that and Altrincham have a bit of an off day then who knows.
"(Matt) Beeby says before each round 'there's always one who makes the first round - why can't it be Leek?' I know we are up against it. But I'm really looking forward to it and so are the fans and the board of directors."
Casswell has also taken on board some words of wisdom from former Kidsgrove Athletic boss Pete Ward who joined Town's managerial set-up in the close season.
He said: "Wardy spoke about Kidsgrove last season who got knocked out in the round after this one. The team were deflated and went on a poor run and never got their season going after that.
"When we do eventually get knocked out we have to make sure we don't become deflated. We're delighted with our cup run and have to take that form into the league and kick on up the table."
Another motivator for Town's players to perform this weekend is the potential for attracting interest from clubs further up the league pyramid.
Former Crewe Alexandra midfielder Dan Shelley, who made his debut in last weekend's FA Trophy win over Radcliffe Borough, is one of a number of players targeting higher level football.
"If you do well against higher league opposition it bodes well for players," added the Blues boss. "I'm not saying I want to lose players. But we have players who have aspirations of playing higher than the level Leek are currently at."
Shelley was one of a handful of players to impress Casswell in Saturday's "lacklustre" FA Trophy victory which sets up a derby clash with Evo-Stik Premier outfit Matlock Town on October 27.
Casswell said: "He (Shelley) did really well. He's not played a lot of football recently. He did a lot of excellent work defensively. He proved his willingness to work, to get down and graft for the club. He missed a couple of good chances but once he's had a couple of games I'm sure he'll be putting those away.
"Matlock away is another great game for the fans. Again there's no pressure as they're a division higher - even though they're not doing brilliantly.
But that's not until the end of October. We're looking forward to going there but have a couple of tough league games coming up first against Chasetown and Coalville."