Python - 211
h=input()*2
d=h/4
q='|'
e=' '
w=e*d
u='\n'
s='+'+'-'*h+'+'
y=e+w+s+u
i=''
v=q+e*h+q
for l in range(0,d):y+=e*(d-l)+'/'+e*h+'/'+e*l+q+u;i+=v+e*(d-l-1)+'/'+u
print y+s+w+q+u+''.join([v+w+q+u]*(d-1))+v+w+'+'+u+i+s
I'd like to note that I took some ideas from JPvdMerwe (Using a string to print once, and the one-liner for that I didn't know was correct syntax in Python)