The ASCII Architect

Ruby, 223 214 bytes

g=$*[0].split(/(?<=[a-j])(?!\+)/).map{|r|r.scan(/(\d*)(-\d+)?([a-j])/).map{|a,b,c|' '*a.to_i+'++--***...'[-b.to_i..c.ord-97]}*''}
puts g.map{|s|s.ljust(g.map(&:size).max).chars.reverse}.transpose.map(&:join).join$/

That was fun. :)

Although it should be quite obvious, I discovered a new way to do these challenges where strings have be constructed from columns: just do them in rows, and transpose the array of characters before joining everything.

g=$*[0].split(/(?<=[a-j])(?!\+)/)               # Split into columns.
       .map{|r|                                 # For each column
            r.scan(/(\d*)(-\d+)?([a-j])/)       # Split into components.
             .map{|a,b,c|                       # For each component
                ' '*a.to_i+                     # Prepend spaces if any.
                '++--***...'[-b.to_i..c.ord-97] # Select the appropriate slice of the tower.
            }*''                                # Join all components together.
        }
puts g.map{|s|                                  # For each column
            s.ljust(g.map(&:size).max)          # Pad with spaces on the right such that. 
                                                # all columns are the same height.
            .chars.reverse                      # Turn into character array and reverse.
      }
      .transpose                                # Mirror in the main diagonal.
      .map(&:join)                              # Join lines.
      .join$/                                   # Join columns.

Cobra - 473

I don't think Cobra's ever going to win one of these :/

use System.Text.RegularExpressions
class P
    def main
        r=Regex.matches(Console.readLine,r'(?<=^|[a-j])(([^a-j]*[a-j])+?)(?=[^+]|$)')
        z,l=0String[](r.count)
        for m in r.count,for n in'[r[m]]'.split('+'),l[m]+=' '.repeat(int.parse('0[Regex.match(n,r'(?<!-)\d+')]'))+'++--***...'[int.parse('0[Regex.match(n,r'(?<=-)\d+')]'):' abcdefghij'.indexOf(n[-1:])]
        for y in l,if y.length>z,z=y.length
        for x in-z+1:1
            for y in l,Console.write(if(-x<y.length,y[-x],' '))
            print

All nice and commented:

EDIT: Just realized this looks suspiciously similar to the Ruby solution. Great minds think alike?

use System.Text.RegularExpressions
class P
    def main
        r=Regex.matches(Console.readLine,r'(?<=^|[a-j])(([^a-j]*[a-j])+?)(?=[^+]|$)')
        # Split into columns
        z,l=0,String[](r.count)
        # Assign the column-array
        for m in r.count
        # Loop through columns
            for n in'[r[m]]'.split('+')
            # Loop through individual letter instructions
            # - within columns
                l[m]+=
                # Add characters to the last column
                    ' '.repeat(int.parse('0[Regex.match(n,r'(?<!-)\d+')]'))+
                    # Any spaces, plus
                    '++--***...'[:' abcdefghij'.indexOf(n[-1:])]
                    # The default column string
                        [int.parse('0[Regex.match(n,r'(?<=-)\d+')]'):]
                        # Sliced to the right length
        for y in l,if y.length>z,z=y.length
        # Determine the maximum length of any column
        for x in-z+1:1
            for y in l
            # Loop through columns so that they rotate to the left
                Console.write(if(-x<y.length,y[-x],' '))
                # Write the character in the current position
            print
            # Insert newlines

Lua - 451

a=arg[1]j='++--***...'I=io.write M=string.match U=string.sub T=table.insert n=''y=0 t={}m=0 for i in a:gmatch('[%-%d]*[a-j]%+?')do b=M(i,'-(%d)')b=b or 0 s=M(U(i,1,1),'%d')s=s or 0 n=n..(' '):rep(s)..U(U(j,1,M(U(i,-2),'[a-j]'):byte()-96),1+b,-1)if U(i,-1,-1)~="+"then T(t,n)m=m<#n and #n or m n=""y=y+1 end end T(t,n)n=''for k,v in pairs(t)do n=#v<m and n..v..(' '):rep(m-#v)or n..v end for i=m,1,-1 do for k=0,m*y-1,m do I(U(n,i+k,i+k))end I'\n'end

Nothing special. It was fun to rename a butt-load of functions for once though. I'll edit the ungolfed code in later.

Try it out here. Sample Output: