Somersault 167 (Harmer 5-64) and 172-for-4 (Lammonby 59, Abell 54*) need another 294 runs to beat essex 462 for December 9 (Critchley 121, Khushi 56*) and 170 for December 7 (Critchley 52, Aldridge 4-36)
The all-rounder Critchley scored 52, to go along with his first innings century, as Essex gave Somerset an improbable 466 runs to win in just under five sessions.
Critchley struck with his first ball to pin Lammonby for a two-hour 59 and also dismissed George Bartlett as Essex need six wickets on the final day to win at Cloud County Ground, Chelmsford for the first time this season – Somerset still need 294 careers .
The Essex Brain Trust had decided not to enforce the follow-on, with the decision hinged on giving the bowlers a longer rest, the tradeoff being less time to remove Somerset.
They had taken their lead of 295 first innings to 310, but lost to Nick Browne in seven overs on the second night.
But the third morning was a meandering session in which Essex seemed in no hurry to declare, albeit against some well-directed bowling. A total of 113 runs were scored in the session with four wickets being dropped.
New Zealand fast bowler Matt Henry covered the Division One highest run-scoring stage earlier in the sixth over of the day.
Paul Walter showed a few moments of intention on his 25 before a low footwork shot was well caught at Gully, before Feroze Khushi fended off a ball that stopped downfield at first slip.
Critchley had shown momentum, as he had during his 121 in the first innings, to push the lead to 450, the loose plan to make a statement.
The former Derbyshire player charged Aldridge over midwicket for six before hitting debutant Shoaib Bashir again over his head.
His half century came on 64 balls and was his sixth of the season. No one in the country has broken fifty more than Critchley’s six in the Championship, though he has only converted three figures once.
After Simon Harmer’s long pick in a post-lunch frenzy, Critchley set off for 52 after delivering a smooth reverse jab to short, fine leg.
Will Buttleman and Ben Allison scored sixes as the lead topped 450, before Tom Westley retired with 55 overs remaining on the day and 151 in the match.
Lammonby and Sean Dickson, who had carried his bat in the early innings, appeared to be in little danger before the latter surprisingly shoved Jamie Porter into midwicket.
Essex tend to put their hopes on Harmer in these situations, especially after his first innings of five for. He was introduced in the seventh over.
The South African bowled 14 unsuccessful overs before Critchley bowled the ball in to make something happen. He promptly did so as a downward drag stayed low and knocked Lammonby, who had hit his third fifty of the season, into the pads.
After two overs from the River End, the leg turn switched to the Hayes Close End and two balls later influenced a lead by Bartlett who was well caught on the first slip.
Abell’s fifty came in 124 balls, but Tom Kohler-Cadmore went lbw to Harmer until the last ball of the day having put on 55 with his captain.