In a war of resources, the best position for a base is the most secret one. Hiding within the deepest and darkest recesses of the Earth will often be your best bet.
Optimal Above Ground Castle Positioning: I've come up with what I believe to be the safest castle positioning possible so far. It might not be as practical in a capture the flag mode... as flags will probably not be located in such locations. Though trying to complete as many criteria as possible will suffice in increasing your chances of survival.
Positioning your castle with the Entrance facing west, and positioned high up forces the opponent to enter from the front unless they wish to be showered on in their little boats by archers on the castle walls from all other directions. Surrounding it with Lava Walling fortifies your defenses even more, making your fort very difficult to penetrate. Opponents coming from the West are at an even greater disadvantage to Cannon fire than from any other side because your cannons will be more effective, and their cannons less effctive, generally reaching around 0 effectiveness unless they manage to build one that can lob tnt over your castle wall in the midst of all the fighting. The bridge made of wood could help with cutting off incoming forces if you were to burn it. You now know also that if you are on the offensive during a war, the best side to mount an above ground normal attack the castle from would be the East because TNT cannons would be at their minimum effectiveness. The best way to penetrate this fortress is through tunneling underground or coming in from the top. Those are much slower than a direct attack and buy anyone in the fortress precious time.
Building an Effective Wall
Through a combination of ideas of my own, ideas of Aninimouse, and ideas presented in a video by Chebuosian, I have come up with what I believe to be one of the most effective walling techniques to date. It will henceforth be known as water walling.
Yep, it's exactly what it sounds like. A wall covered in water. Why? Becasue TNT coming in contact with this type of wall will hencforth be useless. It will be soaked in the water and not explode with enough force to break any blocks. The enemy will either A. Retreat, B. Attempt to Puncture through the Wall [bad idea] or C. Attempt to climb the waterfall [Bad idea].
Assuming first that they have attempted to pursue option B. Let's see what they come up against... 2 layers of stone oh my how annoying. Oh... oh wait there's gravel... how tedious! Let's just get this over with right quic-
Oh my god is that Lava? OH NO RETREAT!
How the heck are we supposed to get through THAT?! Assuming that they then try to climb over the wall... archers stationed on top can pick off those raising themselves through the waterfall slowly.
This is the setup: This wall is 4 thick, just for example's sake. The thicker it is, the more useful to you. The Stone and Gravel can be staggered, or all at once. I chose the second option because it was quicker to build for this example. 2 layers of stone followed by 2 layers of gravel with stone all on top. I then proceeded to create a hollow 2 wide by 10 long trench to hold the lava placed above the gravel [Placing lava above all the gravel unsures that they will not find an unintended opening on the wall], and then held in by more stone. I placed a stone row above the first row of lava so that the water I place does not interfere with it. I placed many water sources and dug a small trench around the bottom of the wall to ensure smooth, aesthetically pleasing water flow.
Edit: Boats... people with boats could utilize a water elevator technique and float right up to the top of your wall! To avoid this, perhaps include an outcrop of stone above the water that a boat would crash into.
If water walling isn't enough:
Staggered Stone/Gravel. Lava Pool on Top. Lava flowing down the sides [works the same as water to prevent TNT cannons blowing up your wall.] gives the added bonus of players not wanting to wade through it. Trying to place blocks into the lava stream to interrupt it... is a bad idea and doesn't work. Trying to use a bucket on the water only turns it into a thicker wall... This walling technique is the best so far.