[tinlizzy]

Once the leaves fall off I'm going to get my conker tree lopped.   Could I use the straight branches to make a trellis or archway?   Would I need to nail them together or use screws?   Or would I just be wasting my time and effort?

[muntjac]

go for it . use the thicker members for arches etc . use nails  , but first drill the hole through the top pole to save it splitting .this also helps with he second and if you use longer nails than needed you can bend them over .use a lumphammer behind to take the shock

[splodger]

i don't know about horse chestnut - but sweet chestnut i use all the time.
for most of the following:

pea sticks and bean sticks
hurdles (gate and woven)
shingles
edges - for borders and raised beds as well as stakes to wedge them in.
post and rail fencing
trellice work
poles for swings and climbing frames - for kids
logs (for wood burner- as the wood spits a bit - but keeps in for ages - so one of the best woods to use in sealed units)

like munty said before - chestnut is a great wood to use - but it will split if not handled properly.

if you have to fix it /something together - pre-drill - or it will split (slowly at first - but the split will run)

it's great because it splits - with a good knife - or wedges (depending on size) you can get just the right shape and size that you want.

if you want to use for post and rail - or something architectural - then it will actually last longer if you remove the bark - this is very easy to go when it's green, but alot more like hard work when dry.

also - the longer you leave it (to season) the hrder it gets - so work it green - use it dry