This fortress.cql was showed in the CQL introduction article of Gady Costeff.

It searches for studies where white finds a position to draw although black has a lot of extra material.

; fortress.cql
(match
:pgn heijden.pgn
:output fortress.pgn
:result 1/2 ; the result is a draw

(position
:piececount A 2 16
:piececount Pa-h7 0 ; white does not threaten to promote
:attackcount K a 0 ; white is not in check
:wtm
:sequence ( ; The structure is a sequence of 4 ply
(position
:attackcount A [nbrq] 0 ; white attacks no black officers
:powerdifference U -1000 -4 ; black's ahead at least 4 pawns
; (P=1 [NB]=3, R=5, Q-9)
:nocheck
:movefrom [KQRBN] ; no white pawn move
:moveto . ) ; no capture (move to an empty square (.)

(position
:attackcount A [nbrq] 0
:powerdifference U -1000 -4
:nocheck
:moveto . )

(position
:attackcount A [nbrq] 0
:powerdifference U -1000 -4
:nocheck
:moveto .
:movefrom [NBRQK])

(position :and (
(position :not :stalemate)
(position :attackcount A [nbrq] 0
:powerdifference U -1000 -4
:nocheck :terminal
:attackcount [qrbn] . 4 100)

)))))
After executing this with "> cql Fortress.cql" there are 122 studies found in the Harold van der Heijden Database V 2015.
NB. When you see {MATCH} in the text, it means that cql has found the position asked for.

Only 5 studies are presented here.