Some of my walks have included going through forests, of which there are many in Snowdonia, mostly fairly small bits but some such as Gwydyr Forest and Coed y Brenin are quite extensive. In the mind’s eye they appear to be good places to wild camp. In practice this is fraught with difficulty. I’ve found one or two acceptable spots in forests, but generally they are poor. Far better places are just by the outside edge of a forest.
Welsh forests are usually situated on fairly rough and steep ground where it’s hard to find a level spot for your tent. The forest tracks are often the only flat bits and there have been occasions when I’ve had to resort to pitching on the edge of one. Then there is usually gravel from the track-building beneath the grass, so your pegs don’t go in easily. If by any chance a forestry vehicle passes you in the morning, they’re early starters; they are unlikely to bother you but you can be sure you’ll get an early awakening.
The Forestry also have a nasty habit of felling bits that you’ve found on your map, so when you get there it looks more like a First World War battlefield.
Forests are generally not grazed, so any flat spot you do find that might accommodate your tent will be thigh high in wet grass and other vegetation, not very pleasant to camp in.
Trees all around you will limit your area of vision, you will feel somewhat hemmed in, with no vistas to be seen, unless you are by a lake or something, while you will be unlikely to get any sun on you in the morning. If it has rained, you will be dripped upon from the trees for long after the rain ended. You will be sheltered from the wind, thus increasing condensation in and on your tent and prolonging it drying out.
Speaking of lakes, they sound idyllic but the more obscure and hidden ones in forests are generally densely fringed by sallows and saplings. “Touristy” ones, like Llyn Crafnant for example, have clear grass areas but your chances of being disturbed increase greatly.
Water is rarely a problem in such forests, they all have streams running through them, but, again, your chances of finding a decent tent pitch near one are very slim.