I had a fairly-megaproject, but I ran into difficulties. The outline was simple - I have constructed a large wall (several z-levels, which is needed to prevent goblins flying over... I'm getting ahead of myself) around the outside of the map (at the very limit of where walls can be built) and planned to install raising clear-glass bridges around the entire outside of the map. The idea was that goblin sieges would arrive, walk onto the lowered bridges, one lever would instantaneously raise the bridges flinging the goblins into the air briefly (but not over the high walls), and then lower the bridges to crush them, and the siege would be over in five seconds, with no mess to clean up. As a bonus, if the bridges were raised the entire map boundary would be impenetrable, and the dwarfs would have free rein over the whole of the map.
However
I had only done one side of the map when I started running into a problem. Most of the bridges worked fine, but some simply couldn't be built, even though the terrain was exactly the same. No hidden items, no forbidden items... the bridges would just suspend construction and I couldn't work out why. A bit disappointing. Those bridges that were installed do work as planned, though.
...due to the failure of that project my next one is to construct a large tower (from ground to sky) each level of which is built of a different pair of materials (floor one thing, walls the other), and the inside of which is filled with shops and "museum displays" of gem windows, unusual rocks, all that sort of thing. That's going ok, and it's interesting to see how different combinations of colours work.