Ello, here are some more random structures. You'll have to forgive me on the first two; I just need to get them off my chest.
1) One way spawn exit
It's always fun to crouch next to your enemy's spawn exit door, and run in while he runs out. Except when someone does it to you, rather than whinging and calling him fag, why not try building something to stop him? This is a building game ffs!
2) A different tunnel design
The other day I was playing on a map that had a shortage of wide flat plates with an excess of long planks and morons. Sometimes it's a good idea to build the long skinny tunnel out of long skinny pieces. The other advantage to this design is you'll actually notice when someone is crawling through it, and they'll be more exposed.
3) A Funnel
Have you ever fallen into a fort's crevice to find you can move but only very slowly? That can be deliberately used as a nastier equivelent of a tunnel. It takes 12 seconds to travel down this tiny example funnel. Once you're in it's also impossible to jump. The sharper angle between them the slower you'll go. Just make sure to trial run it yourself.
4) Ball trap
When you're carrying the ball it doesn't hit other objects above your head. Once you die however, it falls to the ground in the same manner as anything else. As a result you can make a ball trap above your tunell that will make the ball unreachable until it respawns. This idea irsn't practical but it's kind of cool, and probably could be adapted to be less wasteful.
5) A very nasty flag defence
This involves two 'pig pens'; pig pen P1 around the flag and pig pen P2 next to it. There is a ramp into P1, providing easy access to the flag (important for returning stolen flags). The only way out of P1 is on a one-way ramp onto a tightrope above P2. If you fall you're stuck in P2.
The advantage of this is after the flag-captured warning you only have to take a quick machine gun spray at the poor fellow on the tightrope. He'll be an easy target from anyway on the map, because he's suspended on the tightrope.
This takes about 15 minutes to set up, but I believe someone who wasn't a newbie could do it in 7.
6) Replica of Japanese Nanboku-cho era castle
Firstly I better apologise for the poor screenshot. You can only see the main keep. I'll also admit this one takes more than one build round to complete. That aside, it's entirely worth building. From inside your own Japanese castle you can bombard your enemy with stones through the 'murder holes' or pour scalding hot sand on him as he tries in vain to ascend your tower walls. They're fairling resistant to grenades, or block motars (see my other thread for instructions on making them).
The main problem with building a Japanese castle, apart from using several tens of thousands of stone blocks, is they are quite vulnerable to seige. Nothing ruins a good night on sourceforts as much as watching a combine pick the flag out of your malnourished hands.

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Comments? Ethics?