Yes, Level 4 (=Brownies) was the first level I really liked a lot (I'd even go as far as to say loved), although the first three I did enjoy. However, the first had a ton of secrets including a few mandatory ones and also very, very extensive for a start level (not the best first impression), Level 2 was a tuned down SHS level that I felt was much better than the original SHS version but many do not appreciate map reuses of any form, and while I felt Level 3 was solid, it was unremarkable (though Grey Area is a cool name

). I really liked the brown textures, length, and difficulty of Brownies (Floor 4); in fact, it is probably a top five map in the set of mine. It was very big and extensive, but done so in a better way than Floor 1 IMO and also more appropriate since the player is into the mapset some now. (Note that I'm leaving Floor 2 out of consideration because as I mentioned, a reused level can turn some people off right away, despite it not bothering me.)
I do remember the completely symmetrical levels were a low point for me, mainly the non-boss/Rampart ones (which fortunately reduces to just Level 8 but yes, that was one of the weaker levels). I'm glad that otherwise no levels except the boss ones and Ramparts (which has to be and I enjoyed it) were completely symmetric and you even strayed from the boss level symmetry on Levels 16/18. I know ack has made some completely symmetric levels that didn't bother me as well, but Level 8 didn't feel quite right. And I clearly remember Levels 11 and 12 and how I felt they were different and overall somewhat inferior to the rest.
Level 16 reminded me of one of ack's levels, as you mentioned, and in a good way, but I can't remember what level it was. I think it was an Archives one, as I seem to remember is was for
Spear of Destiny as well.
I don't remember the secret levels that much aside from that both were very difficult, even harder than most normal levels, and I liked them both. But overall I know I loved Levels 4 and 16 and I at least quite liked all but: 1 (you should not go total full scale on a starting level), marginally 5, 8, marginally 10, 11,and 12 (note I let Level 2 get by as I personally didn't mind the reused level). I put in an excellent vote myself because all in all, I think it's at least close to that as a whole, and I found the improvement from SHS to be very, very strong. If I recall I had played virtually all of This
Spear before the final version was even released, and I never even played half of SHS. That's a big jump, no doubt about it.
