Back to Basics
Having replayed Espi's Back to Basics recently here are some thoughts on its gameplay that I neglected to note at the time. This wad is pretty much flawless in terms of - well, I don't have a word, but stuff like cohesion and interconnection of adjacent areas, and glimpses of places you can't reach yet that draw the player in, that kind of thing. However it can be convoluted and you might find yourself going off on wild goose chases if you don't go through each map's sections in the right order.
It seems impossible to avoid having to trek through half an empty level at some point in this map but there are two ways you can go and one is better than the other.
If you explore the start fully you will find blue and yellow doors, then the blue key seen through a window next to a red switch. The first key you find will be the yellow. At this point do not turn back to the yellow door but keep on exploring the area you found the key, there is a not hidden but not obvious switch to open some bars which allow you to carry on.
It's important to do it in this order rather than go back for the yellow door. If you go back, you will go through an extended section eventually leading tothe red key on a pillar. You are meant to jump off a ledge and get the red key, but you can't do this until you have removed the barrier surrounding it, which is done with a switch nearby. Once you have pressed this switch you will have to then re-traverse almost the entire map to get back to the ledge to jump off. It's much better if you carry on past the yellow key to find the switch opening the barrier around the red key, before going back for the yellow door.
One downside of doing it this way is that you will reach the backpack secret late, but you will get to the blue armour secret earlier.
Explore the start fully. At some point inside some flesh-lined tunnels there is a short drop down to a door into the second part of the base. Do not go down here until you're sure there's nowhere else to go. In particular you should have both a chaingun and a rocket launcher. Once you drop down there's no way back up until you have the blue key and it's a long way to go without those two weapons, I think.
(This map has one of the best examples of disjointness. If you ever wonder what I mean by disjointness, look at how the main blue door in the middle separates the map into two completely different parts that have no effect on each other. The part beyond the door has no influence on what came before it, and once you reach it and open it, there's almost certainly nothing left behind you except a few resources you might not have picked up.)
Two of the monsters in the teleporters protecting the exit will not teleport out if you go over the area where the exit is hidden before lowering the floors. Unfortunately the surrounding linedefs are single-use and can be used up by your walking over them but the floor having nowhere to move down to yet.
I picked up the blue armour hidden near the start, and it lasted all the way through the map. I finally decided it had run low enough to replace when I got the soulsphere near the end. At that point I ran back a long way through the empty level to fetch a green armour I'd left behind. After slogging all the way back, what should I find at the very next fight - a second green armour. Try not to do this yourself.
But if you do decide to backtrack anyway, once you've got the blue key and opened the door and jumped past the baron through the window, don't forget to press the switch to open the bar from the other side, else you'll have to go all the way around and jump again.
The final level is a central chamber surrounded by a series of rooms, staircases and corridors which almost encircle it. If you do not want to enter the chamber as soon as you see it you can go off to one side and encircle the level clockwise. However, I think it's better to briefly enter the chamber, go for the door to the west and circle the outer part anticlockwise, which gets you a backpack sooner so you can pick up all the ammo as you find it. Note that you can shoot a button behind the caged cacodemon to open the bars opposite it.
Note: don't touch the backpack in the central chamber until you're ready to wake the level boss up! Actually this might not be a bad idea as it would clear some of the other copious number of monsters in the chamber, but also it might well get caught on a knobbly bit of architecture in there and get chewed to death by demons causing the level to end before you've finished it.
This level in particular has a lot of rather thin walkways not aligned with the grid axes which can make it rather hard to traverse - you get caught on walls, the wall sliding code works much better with axis-parallel walls - and make the monsters appear to move rather oddly, as they only move in the eight cardinal compass directions.