I'm personally not one for strong-arming anyone to play specific things. Note to everyone concerned that without a cleric, healing will behard to come by. Note to them that if they want a lot of straight out knock-down drag-out fights that a monk isn't as effective as a fighter. Then let them choose. The only thing in the story they get to control is their own character, so it should be the one they want (within reason). If they have no cleric, make sure healing magics are not too difficult to get elsewhere. If they have no fighter, don't throw quite as much heavy combat at them.
Not being able to pull stats out of the air isn't much of a detriment if you do enough preparation beforehand. In this city environment, that means having stats for guards, commoners, the generic enemy, shopkeepers, and various local folk already done up before the session starts. If you are good at improv acting, adding personality to these stats later won't be a big deal.
As for railroading, there are two approaches that come to mind:
1)Don't bother creating a "plot". Instead, create a situation. In a plot, you have already determined the waypoints - the characters must go to point A and do X, then got to point B and do Y. If they don't do these things, the plot falls apart. If you haven't prepared anything outside the plot, you either have to "wing it", or railroad them back into the plot. If, instead, you create a situation, without predetermining the solution, the party makes their own plot as they work to find one.
In your case, that means making up lots of stuff about this city and it's attackers. Detail anything that might affect the outcome of the situation, and let things flow from there. Setup the physical layout of your city. Figure out who the town leaders are. Figure out who the enemy leaders are, etc. If you know the NPCs, you know how they will interact with each other, the situation, and the PCs. From there, it's all improv acting
2)Know your PCs and players, and make a plot that you know they won't resist. It isn't railroading if you know they'll gladly bite at the hook. This doesn't work to weell if you don't know the PCs, if the PCs aren't strong personalities, or if the players aren't the most consistent. But, if the player has made up a background story that includes a beloved little sister, you know he'll take the bait if you put that sister in peril
The secret here is to make sure that every PC has a reason to go along. Don't assume the party fighter wants to go save the cleric's sister. The fighter needs his own motivation. Few folks like to just "tag along".
All in all, for the general idea you've got, the first method I mention is probably the more natural choice.
Not being able to pull stats out of the air isn't much of a detriment if you do enough preparation beforehand. In this city environment, that means having stats for guards, commoners, the generic enemy, shopkeepers, and various local folk already done up before the session starts. If you are good at improv acting, adding personality to these stats later won't be a big deal.
As for railroading, there are two approaches that come to mind:
1)Don't bother creating a "plot". Instead, create a situation. In a plot, you have already determined the waypoints - the characters must go to point A and do X, then got to point B and do Y. If they don't do these things, the plot falls apart. If you haven't prepared anything outside the plot, you either have to "wing it", or railroad them back into the plot. If, instead, you create a situation, without predetermining the solution, the party makes their own plot as they work to find one.
In your case, that means making up lots of stuff about this city and it's attackers. Detail anything that might affect the outcome of the situation, and let things flow from there. Setup the physical layout of your city. Figure out who the town leaders are. Figure out who the enemy leaders are, etc. If you know the NPCs, you know how they will interact with each other, the situation, and the PCs. From there, it's all improv acting

2)Know your PCs and players, and make a plot that you know they won't resist. It isn't railroading if you know they'll gladly bite at the hook. This doesn't work to weell if you don't know the PCs, if the PCs aren't strong personalities, or if the players aren't the most consistent. But, if the player has made up a background story that includes a beloved little sister, you know he'll take the bait if you put that sister in peril

All in all, for the general idea you've got, the first method I mention is probably the more natural choice.
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