So my plan here is to do a tower defense one shot where basically there's a crystal the PCs will have to defend and they will level up throughout the waves. I was thinking for the terrain it could be a really high aztec-ish tower and the hordes of demons would at one point become unrelenting. For leveling up I was thinking about the players having 4 sheets for their characters for a quick way of leveling up with them starting at 5 and going 10, 15, 20. What recommendations would you guys add? Change? Take away https://100001****/ https://1921681254.mx/?

Going through all 4 tiers by combat in a one shot will be next to impossible. Fights, especially at higher levels, can take really long time.

I even think, most one shots are not considering levels at all, because they are over after only this one session.

Yeah, thats a tall order for a one shot.  Obviously you wouldn't be using XP based levelling.

I guess argue that if they hold it for a certain time they can trigger some special ability the tower or crystal has to either grant them more power or time stop while they rest.  Alternatively you simply note the attacks are a few years apart each and the defenders gather every time, having levelled up from other adventures while the armies of hell have rallied stronger minions for the next incursion.

I'd also consider something more interesting than the top of an aztec tower.  It just feels boring.  Maybe a 3 floor tower set against a canyon side with a river flowing by one side.  Put the crystal in the middle room and then they'll be defending attacks from the top and bottom, have a room above and below the crystal to fight back from and retreat to when the hoards reach the tower, some might try to sneak in by the river and throw up grappling hooks to climb and attack from the top, stealthily, another encounter might see fliers and yet another might see a direct tunnel dug in from the side of the canyon.  It'll keep them on their toes as opposed to just moving about the top of a square pyramid. 

I would have the waves of combat be years apart.   Weeks or months tend to make the civilians leave the area, years is enough that people will stay till it is too late to leave.

I would make an entire castle, with at least 3 layers of defense - outer wall, inner wall, then a keep.

Note, hordes tend to take forever in a fight.   Especially if the players have minions.

I suggest you do the 'small part of the war' bit where the players are given one specific part of the keep, for example the "outer North-North-West Wall" and then when things go down, they retreat to the "inner North-West Wall", then they retreat to the "Keep's North Wall".

This lets you control the flow of the battle even if the players are winning or losing.  That is, if they win, you can say the guy next to them is losing and they need to help out (i.e. increase the planned number of monsters), or the other way around.

It also means you can describe armies of thousands when they are only fighting tens.

Make sure your players are experienced. Newbies won’t have any idea what to do with T3 and 4 characters. Too many new abilities and spells will get really confusing. You might even do levels 1-2-3-4 if they’re really new. 

As others have said, this will need players with experience playing up to Tier 4, more time than a typical one-shot is given to accommodate four big combats, and a whole lot of prep work. I think it would probably be better as four separate one-shots, tbh, or at least two with two fights each, but it could work as one long day of gaming

The other thing you might want to consider changing is the characters' levels. In terms of when you get class/subclass abilities and such, 5-10-15-20 is actually kind of awkward. Something like 3-7-14-20 might make more sense

All that said, it is a cool idea if you can pull it off. That last wave is gonna be very Helm's Deep. Assuming the players know what they're making their characters for, it could result in some very interesting builds and choices

Maybe consider an overall story concept to explain the big level jumps too (i.e., the crystal or its protector keeps summoning the party when needed and then returning them to their world between fights, but time flows differently between the realms etc etc so it's been months or years for the party but only hours or days in the tower, that sort of thing. That would also give the players opportunities to bring in a new character if one dies or they find their initial build isn't working the way they wanted it to).