A structured note that builds up on a tunnel note, by an addition of mechanism enabling the holder to accumulate higher coupons, layers upon layers, like in a tower depending on which range the underlying asset sticks to over a span of a specific period. This involves foregoing part of the market coupon against a long position in a portfolio of nested barrier options (nested double-no-touch contracts).

The “tunnel” may take two forms: an expanding tunnel (the range of fluctuation widens over time) and a contracting/ shirking tunnel (the range narrows down over time).