Sentinel is the name of the Axalon's automated defense system which proved to be extraordinarily effective. To put it simply, most battles near the Maximal Base ended by someone activating Sentinel. This caused the Maximal device to be highly coveted by the Predacons.
The Sentinel System itself consisted of an extremely powerful energy shield, and the simultaneous activation of every weapon attached to the Axalon. Despite its power, the shield can be disabled under unusually heavy fire. It had other limitations as well; Blackrachnia once built a device that was capable of penetrating the shield for a short amount of time (Other Voices Part I). In the same episode it was shown the shield does not extend its protection into the ground as Inferno was able to dig a tunnel to bypass it completely (Although this was not a significant drawback as Inferno was the only Predacon with a beast mode capable of such an approach). Finally in order to have the shield operate at full power all internal defenses had to be taken offline.
Despite these drawbacks, it was still a lot better than the original shield system found on the Axalon, which had quickly failed while battling the Darksyde in orbit and could be permanently disabled if hit while its circuitry was damaged.
Beast Wars cartoon
Voice actor: Ian James Corlett (English) ? (Japanese)
The Sentinel system was developed by Rhinox. Initially, the system went haywire when Rattrap and Dinobot got into a brawl, attacking the Maximals themselves due to Rhinox having yet to install the 'Friend or Foe' program, but Rattrap was able to shut it down without being killed.
At one point, Megatron attempted to disable Sentinel by using a clone of Dinobot to infiltrate the base, but this plan was flawed due to the clone lacking Dinobot's Maximal energy signature or knowledge of his deactivation code; the clone's apparent success was only caused when the original Dinobot returned to the Axalon, allowing Dinobot to catch Megatron off-guard after luring him to the base by posing as the clone.
After the Axalon was dragged off a cliff and crashed into the lake below, the Maximals tried to retrieve Sentinel's CPU from the wreckage. Rattrap piloted a submarine into the wreckage and brought the module to the surface. Despite their efforts, the Predacons attacked, and the module wound up in Megatron's clutches. Given the need for the Maximals to focus on defense of the Ark, this forced the Maximals into an almost entirely defensive footing for the rest of the war.
However, even Sentinel proved ineffective against the onslaught of the Vok emissary Tigerhawk, who destroyed the Darksyde and Sentinel with it simply by landing. Of course, he was surrounded by a ball of energy moving at an incredibly high speed— the energy having previously disrupted the DarksydeTemplate:'s systems to shut down its power—, so that probably helped.