- "But for this week for added value, we've introduced...The Sentinel. Ever present and menacing, it sweeps the floor in a wide arch. Now, a timely dash could get the robot to the end zone safely, an untimely one could get it spiked!"
- — Philippa Forrester introduces The Sentinel in Heat G of Series 2
The Sentinel was a short-lived House Robot that was introduced halfway through Series 2 of Robot Wars. Essentially a modified JCB, it first appeared in Heat G as an obstacle in the Gauntlet stage, occupying the left-hand route, and appeared in every heat thereafter except Heat J as well as the Semi-Finals. In the Grand Final and some of the end-of-series Grudge Matches, it also appeared in the arena as an additional hazard, located in a corner of the PPZ and effectively acting as a sixth House Robot.
Because of its status as a Gauntlet 'obstacle' rather than a conventional House Robot, The Sentinel was retired at the end of Series 2, as the Gauntlet stages were dropped for Series 3 onwards.
The Sentinel was a modified JCB designed to resemble a robotic arm, being decorated with ornamental tanks, pipes and cables, and the hydraulic arm modified to feature a large spiked club at the end. The club was intended to push, block and pin robots against the wall or arena floor when they attempted to pass it, as well as to cause damage to their armour, and steer them into other House Robots or a nearby pit. Uniquely, The Sentinel was operated inside the arena rather than in a separate control area, the only House Robot to be controlled this way. A large square-shaped enclosure surrounded The Sentinel's tracks, preventing it from being able to move outside of its designated area during Gauntlet runs.
Initially rotating at 180 degrees, The Sentinel's arm was later made to rotate a full 360 degrees for the Semi-Finals; at the same time, the arm and the club were also made to lift and lower independently of each other. As a result, it was now able to occupy both the left-hand and centre routes, posing an additional obstacle for competitors which chose the latter route. On the centre route, The Sentinel could block, lift and push robots off the left-hand platform, with the intention of forcing them to take the see-saw or otherwise position them over the edge. The latter would often result in the affected robot toppling into the pit next to the platform.
The Sentinel first appeared in the Gauntlet stage of Heat G, paired with Dead Metal. In its first run, it succeeded in pinning reigning champion Roadblock against the wall as it attempted to negotiate past, although Roadblock eventually escaped and reached the end zone. It briefly nudged Onslaught against the wall during its run, before the latter got past easily due to its small size. However, The Sentinel's arm caught Onslaught and forced it into Dead Metal, impeding its progress and allowing it to eventually steer Onslaught into the pit.
In Heat H, The Sentinel was unable to block Cassius, Loco or Sting during their runs as they successfully negotiated through the space under its arm, although both Loco and Sting were caught and pitted by Dead Metal not long afterwards. Additionally, Wizard and Rampage encountered problems before they were able to reach it after taking the left-hand route during their runs. The Sentinel pinned Prometheus against the wall in Heat I, after the latter changed routes mid-way through its run. It succeeded in puncturing through Prometheus' armour with the spiked arm, and allowing Sir Killalot to close in and push it back towards the starting line.
While it was not used in Heat J, The Sentinel returned in Heat K, now paired with Matilda. It failed to prevent Demon, The Mule or Plunderbird 2 from passing it during their runs, although Demon was subsequently attacked by the House Robots, and Plunderbird 2 deliberately drove into the pit immediately afterwards. One of The Sentinel's decorative pieces fell off as Enzyme negotiated past it, with Matilda pitting it straight after it drove past.
The Sentinel performed much more strongly in Heat L, impeding all four competitors which attempted to negotiate its route. It pinned Vercingetorix against the wall and immobilised it after jabbing through its front end, although not without having one of its spikes damaged in the process. The Sentinel also helped Dead Metal force Haardvark into the pit and block Havoc as it attempted to run past it through the middle, before steering Flirty Skirty towards the pit, allowing Sir Killalot to lift and drop it in.
The Sentinel returned for the Semi-Finals, now raised higher and with the capability of lifting and rotating its arm at a full 360 degrees. With these new capabilities, it blocked Mace during its initial run in the first Semi-Final, forcing it to retreat and attempt the see-saw, before playing a pivotal part in its subsequent Race-Off against Panic Attack. The Sentinel blocked Mace as it attempted to negotiate the platform, before lifting it against the railings and eventually forcing it to topple into the pit, allowing Panic Attack to progress to the Pinball trial. In the second Semi-Final, it blocked Roadblock again, this time succeeding in preventing it from travelling further despite sustaining damage from Roadblock's circular saw. The Sentinel also blocked Haardvark after it changed routes and attempted to make a run past it, before Matilda dragged Haardvark back to the start as time ran out.
The Sentinel made its arena debut in the Grand Final, appearing in both of the eliminators, although it did not play any significant part in either of them. However it played a major role in the third-place playoff between Roadblock and Killertron, pinning the latter in the PPZ after Roadblock pushed it into it and allowing the House Robots to attack it. This was The Sentinel's last notable appearance in the main series, although it later appeared in many of the Grudge Matches shown at the end of the Second Wars, especially in the Judgement Day battle between Mortis and Napalm.
There, it thrust its arm down on Mortis' underside after it had been flipped over by Matilda and Sir Killalot, causing damage before Sir Killalot carried Mortis towards the Flame Pit. This was The Sentinel's final appearance in Robot Wars, and it was retired after this, presumably following the producers' decision to abandon the Gauntlet and Trials for the Third Wars.
- As it was introduced halfway through Series 2 and removed after the end of the series, The Sentinel was the shortest-lived House Robot in the show's history, and the only one to be retired during the original series' run.
- The Sentinel was the only House Robot which was unable to move outside of its position through any method.
- Due to The Sentinel's nature, it is probable that it is by far the heaviest and tallest of the House Robots.
- The Sentinel was the only robot of any kind that was controlled from inside the arena. Its appearances marked the only occasions where a human operator was present in the arena whilst robots were competing.
- The Sentinel faced Roadblock four times, more than any other competitor. It was present during Roadblock's Heat and Semi-Final Gauntlet runs, its Grand Final eliminator against Cassius, and its playoff against Killertron. However, it did not make any contact with the defending champion during either of those arena battles.