There are no publicly available tutorials purely on CTF gameplay yet, so I decided to make one. I suggest you play CTF-Coret and CTF-Cybrosis][ (bonuspack 1) to better understeand the examples I will give below.

CTF is in a way different from DM with floorplan and design but there are also a lot of similarities. In a DM map you have to be able of play with the opponent, hunt him down, predict the opponent's next move, and that opponent will have to be able of knowing that he's being hunted down, and he has to be able to escape, too.

In CTF you have the same thing, you need to be able to predict the next move of your opponent(s). If you are in defense for instance, you need to be able to control your base. For example, if you are on defense in CTF-Coret, you can easily find out from which direction the enemy is going too come. The doors warn you when someone tries to enter the base trough the lower way, the glass floors let you see if someone is trying to come from the upper way, if he escapes with your flag over the high route you will be able of pinpointing his position because you hear the jumppad, and if he goes low you hear him going trough the doors again. But now comes the interesting part: the enemy knows you heard it, so he will adjust his route, and in case of the jumpad with flag situation he will jump off of the high floor again, and now goes low. But then you notice he's not going to come over the upper route after you waited for a few seconds, so you run to the low way, which is easy to reach, yet far enough to give the enemy a far chance of escaping. And then again you can hear the second door. You need to be able of reaching the other routes in very little time- there has to be enough connectivity, but less than DM of course. You always have to be able of predicting the next move of your enemy, and while doing that, giving the enemy a fair chance of doing the same...

A lot of people also think CTF is running from point a to point b (flag to flag): that's not true- it's getting from a to z by going trough points b c d e f g h, then change route and go to i j k l m n o p, then predict the enemy (you know he is defending on the high route for instance, so you wont take that route), by going to q r s t and then attacking the enemy base by points x y, and finally taking their flag at point z and going all the wayback through all the points to your base. Nobody is interested in a giant, long, boring corridor (or worse, a cube) with 2 flags at each end.

That's a problem you see in a lot of CTF maps that are too hard to learn. As far as connectivity is concerned, a lot of maps seem to be built up from 3 cubes: 1 cube for blue, 1 for red, and 1 in the middle, and then connecting them with each other. That sucks. You need to be able to switch routes without first going to 1 of those 3 major places.

If A is on defense, he will see enemy B attacking the base through the lower route, B will warn A twice by looking through the hole in the wall and by the door sound. A will then go to point X and there they will meet each other.

A better example:
Red Attacker A (comes from route from previous example and follows red line) meets Blue defender C and frags him. But C knows someone is coming and warns his team while he respawns. Now second defender B knows the enemy is going to come low, because 1 heard the warning from C, 2 sees A running trough the glass he has on the upper route (number 1), and 3 he hears the door in the low route (number 2). So B runs back to his base, but in the meantime A already has the blue flag, and leaves the base over the high route (purple line), by doing this he warns the other people from the blue team cause he used the kicker, which makes a warning sound (number 3). So now blue knows the enemy flag carrier is going high and the respawning blue people gather at the exit of the high route and intercept the enemy flag carrier.

And now to make it a bit more advanced, A knows that C most likely warned his team after he fragged him. So A switches route to mislead the enemy (in this case blue defender B which is already running to the lower side of the base cause of the warning from C), and A now goes over the high route since he can easily switch routes. There by he surprises B who didn't expect an attack from the high route and A can much more easily steal the flag. This makes the map interesting: the playing around, the right balance, and the right connectivity.

To make the concept a bit clearer, here's an example from CTF-Cybrosis][:

Red defender B is on defense and is standing on the side platform. Blue attacker attacks the base over the upper route, through the glass tunnel. Now B can see A running trough the tunnel, so B knows A is going to come out in the red base in a few seconds through the high route (duh). But A can also see B through the glass, so A sees that B is running back to his base to wait on him. So instead, A now changes his route which is easy possible in this map, and instead translocates trough the hole in the ceiling of the glass tunnel (number 1) and now attacks through another way. Or, A runs back to number 2 and switches routes through a hole in the floor there.

Now a flagcarrier example

Blue guy A just took the red flag, and he goes high. Red guy B just respawned outside his base, and sees how A with the red flag runs trough the glass tunnel on the upper route (1), so he now knows the position of the flag carrier and warns his team that the FC (flag carrier) is going out high. FC A gives away his position again by going through the door (2). And he knows he has probably been located already so he decides to switch routes right before he makes it to the middle of the map where there is a more space, and a greater possibility of facing an enemy (The green X / enemy B will possibly be waiting there for the FC, or a few of his teammates will because they heard the warning from B). FC A now jumps through the hole in the floor (3) and goes low. Notice the little puddles of water at the low route, their sound can warn the enemy too.

The general thing to remember in CTF maps: multiple routes are important, and they need to connect to each other, but not too much because you don't want a labyrinth of 5000 different routes. Other things that could be interesting are routes that can only be entered from 1 side, or for which you need to use the translocator. Like a hole in a wall somewhere so you have a shortcut (for instance in CTF-Hydro16). This feature could be useful for misleading and escaping with the flag easily, too (hammer jumping, for example). It's all those different possibilities being combined that make a map interesting. Maps like CTF-Niven are boring for that, since the gameplay is too much 'run forward and shoot.'