Okay, I felt guilty about forgetting to run it in the last gameday so I had a look through and I have some feedback for you.
Overall theres some really cool ideas here, but you're trying to cram far too much into a small map. Everything but the cart path is a maze of long narrow corridors. The layouts of these side routes aren't complicated, but its very hard to tell which routes go where - you can follow a passage and end up at a dead-end door, or the opposite side of the map, or the other direction to where you're going. It also makes almost none of the map feel safe: wherever you are, its very likely a scout or spy or pyro will hop out of an un-noticed side route and shiv you in the back. I would remove the underground tunnels entirely, honestly: they just let you skip past so much of the map. Either your team is split up to fight in two places at once, or all the tunnels are is a free route for someone to sneak behind the enemy lines.
The general layout of first, with the cart crossing the bridge and height advantages on each side, is really cool - my problem is that it's not very wide. Your most direct comparisons here are badwater (look how wide badwater's first point is!) and frontier - which has a whole extra set of routes around the buildings. I would make the first point much wider - maybe 256-320 units wider. This gives the attackers a bit more space to attack through, makes the lowest level less tight and killbox-y, and means that spamming the bridge is harder and less useful. The height advantages are also very pronounced: I would suggest making the left side (in the image above) only 128 or so units higher than the bridge. That should make it easier for the attackers - remember this area should be reasonably easy for them.
I like what you're going for here - something like Badwater or Boundary? But there's problems with it, mainly that its very difficult to get from the lower level (the marked X) to the top. Which is the shortest route here? Back into spawn? It forces teams to split into two: half push the cart, half go up to the top to clear out the defenders. Badwater does similar things at both A and B, but at A its very easy to get up to the top, and at B the defender's position only overlooks the point itself: here the defensible point locks down the entire map so far.
Something that happened a lot when I played a1 was that a series of sentries would be set up around the corner, meaning that fighting on the top level is difficult: edge around the corner and you get blasted by a sentry.
I would consider rebuilding this area fairly significantly. I would remove the upper level entirely, add the first point below where it is now, and build a fun fighting space there - which could include adding a more notable choke afterwards so sentries way way back. Think about the break between badwater A and B: it splits the two areas apart effectively without being a single big wide corridor (like you have now) or a chokepoint).
This would also let you take out the very long straight corridor in the image above - Very long straight corridors are bad. Unusually complicated tunnel systems are bad. I know that sounds like a contradiction, but the answer is: if you find yourself making a very long straight corridor, you've made a mistake elsewhere. Look at that thing! There's nothing that long and straight (and, let me say, boring) in all of tf2.
I like the twisty nature of this point, but look at the defenses down at the bottom! I've highlighted all the seperate areas where one needs to keep an eye out for a sniper or sentry: I count ten different ones in this image, plus two at least behind where the camera is! And the side routes lead down to several of these, so defenders are in the same position. You need to simplify the end of this picture, I think. For a reference for what sort of thing you're trying to do here, look at Badwater B-C - and how simple it is. There's one flank which goes around the side above the cart path, and one building for defenders - without an entire seperate underground area, and a jump route, and two bridges, etc etc.
Any one or two of the things you're doing here would be really cool. Like I said, these are good ideas, you're just doing loads of them.
For me the main problem with last is that its got very high heights and that the cart path is very narrow, and directly under the spawns. Soldiers spawn, and are literally right on top of the attackers ready to spam down without much chance of reprisal. It's difficult to get up to the top without scrambling up the crates or finding one of the long side routes, and once you're there you have to go up AGAIN to get to one area - a sentry gun on that second bridge is very well-protected against spies, and down at ground level the first bridge gives it cover, so its well-protected from soldiers and demomen.
I'm not sure what the most direct analogy for what you're doing here is. Look at Frontier last, and also look at Badwater, Goldrush, etc (and thundermountain 2.2): see where the sentry spots are and what their weaknesses are. See where the flank routes are: what the risks and rewards are of them?
I hope that was helpful! This is a great effort for an early map, and I very much hope to see some of the clever ideas - the bridge at first, the twisty second, the narrow low last - given more consideration so as to work as well as they deserve to.